See also:

• CYTOLOGY (from huros, a hollow vessel, and X6yos, science)

It is an integral See also:

• PART

See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

And all three structures may be converted one into the other (Schimper). The chloroplasts are generally distinguished by their See also:

• GREEN, A
• GREEN, ALEXANDER HENRY (1832—1896)
• GREEN, DUFF (1791—1875)
• GREEN, JOHN RICHARD (1837—1883)
• GREEN, MATTHEW (1696-1737)
• GREEN, THOMAS HILL (1836-1882)
• GREEN, VALENTINE (1739–1813)
• GREEN, WILLIAM HENRY (.1825–1900)

Those chromatophores which remain colourless, and serve simply as starch-formers in parts of the plant not exposed to the light, are called leucoplasts or amyloplasts. They are composed of a homogeneous proteid substance, and often contain albuminoid or proteid crystals of the same See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

Tamus communis, See also:

• ASPARAGUS

The outer layers are denser than the inner, the density decreasing more or less uniformly from the outside layers to the centre of hilum. The outermost, newly formed layer is composed of a more homogeneous, denser substance than theinner one, and can be distinguished in all starch-grains that are in See also:

• PROCESS

In the yeast cell it accumulates and disappears very rapidly according to the conditions of nutrition and is sometimes so abundant as to fill the cell almost entirely (Errera, 1882, 1895: See also:

• WAGER (derived, through Fr. wagier, gagier, from Lat. vadiunz, a pledge)

The nucleus is regarded as a controlling centre of cell-activity, upon which the growth and development of the cell in large measure depends, and as the See also:

• AGENT (from Lat. agere, to act)

It appears to be a combination of a protein with nucleic acid. Recent researches have shown that the nucleic acid can be broken up by chemical means into a number of different compounds or bases. The results at first obtained were very confusing and seemed to show that nucleic acid is very variable in constitution, but thanks to the work 4f Schmiedeberg and Stendel (See also:

• GERMANY
• GERMANY (Ger. Deutschland)

1, E, F) by a See also:

• FUSION

In multinucleate cells the division of the nucleus is independent of the division of the cell. Nuclear division may be indirect or direct, that is to say it may either be accompanied by a See also:

• SERIES (a Latin word from serere, to join)

Mitosis.—In indirect nuclear division the nucleus undergoes a series of complicated changes, which result in an equal division of the See also:

• CHROMATIC (Gr. xpcoµaruc6s, coloured, from xpwµa, colour)

This is called the nuclear See also:

• PLATE

The spindle arises partly from the cytoplasm, partly from the nucleus, or it may be derived entirely from the nucleus—intranuclear spindle—as occurs in many of the lower plants (Fungi, &c.). The formation of the spindle begins in the prophases of division. A layer of delicate filamentous cytoplasm—kinoplasm—may collect around the nucleus, or at its poles, out of which the spindle is formed. As division proceeds, the filamentous nature of this cytoplasm becomes more prominent and the threads begin either to converge towards the poles of the nucleus, to form a bipolar spindle, or may converge towards, or radiate from, several different points, to form a multi-polar spindle. The wall of the nucleus breaks down, and the cytoplasmic spindle-fibres become mixed with those derived from the nuclear network. The formation of the spindle differs in details in different plants. The significance of this complex series of changes is very largely hypothetical. It is clear, however, that an equal quantitative division and See also:

• DISTRIBUTION (Lat. distribuere, to deal out)

The reduction is brought about simply by the segmentation of the spirem thread into half the number of segments instead of the normal number. In order to effect this the individual chromosomes must become associated in some way, for there is no diminution in the actual amount of nuclear substance, and this leads to certain modifications in the division which are not seen in the vegetative nuclei. The two divisions of the spore mother cell in which the reduction takes place, follow each other very rapidly and are known as Heterotype and Homotype (Flemming), or according to the terminology of See also:

• FARMER, RICHARD (1735–1787)

2, H. I, J, K). This V has been variously interpreted. Some observers consider that it represents a longitudinal half of the See also:

• ORIGINAL

In the higher plants, after the separation of the daughter nuclei, minute granular swellings appear, in the equatorial region, on the connecting fibres which still persist between the two nuclei, to form what is called the cell-plate. These fuse together to form a membrane (fig. i, C, D) which splits into two layers between which the new cell-wall is laid down. In the Thallophytes the cytoplasm may be segmented by constriction, due to the in-growth of a new cell wall from the old one, as in Spirogyra and Cladophora, or by the formation of cleavage furrows in which the new cell-wall is secreted, as occurs in the formation of the spores in many Algae and Fungi. Cell budding takes place in yeast and in the formation of the conidia of Fungi. In a few cases both among the higher and the lower plants, of which the formation of spores in the See also:

• ASCUS (Gr. &vu5s, a bag)

In the Fungi it is usually composed of a modified form of cellulose known as fungus cellulose, which, according to Mangin, consists of callose in combination either with cellulose or pectic compounds. The growth of the cell-wall takes place by the addition of new layers to those already formed. These layers arc secreted by the protoplasm by the direct apposition of substances on those already in existence; and they may go on increasing in thickness, both by apposition and by the intussusception of particles probably carried in through the protoplasmic fibres, which penetrate the cell-wall as See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

Staining reagents can also be used to differentiate lignified cell-walls. Cuticularized or suberized cell-walls occur especially in those cells which per-form a protective function. They are impervious to water and gases. Both cuticularized and suberized membranes are insoluble in cuprammonia, and are coloured yellow or brown in a solution of chlor-iodide of See also:

• ZINC

In many of the Fungi the non-motile male cell or nucleus is carried by means of a fertilizing See also:

• TUBE (Lat. tuba)

In the Vascular Cryptogams the structure is much the same, but a more or less spherical mass of cytoplasm remains attached to the posterior spirals, and a large number of cilia are grouped along the cytoplasmic anterior portion of the spiral. In Zamia (fig. 4, A), Cycas and Ginkgo they consist of large spherical or oval cells with a coiled See also:

• BAND

Even in the multinucleate oosphere of Albugo bliti the nuclei fuse in pairs; and in the oospheres of Sphaeroplea, which may contain more than one nucleus, the egg nucleus is formed by the fusion of one only of these with the spermatozoid nucleus (Klebahn). In the higher Fungi nuclear fusions take place in basidia or See also:

• ASCI

3, A, B ). A double fertilization thus takes place. Both nuclei are elongated vermiform structures, and as they enter the embryo sac present a twisted appearance like a spermatozoid without cilia (fig. 3, A, B). It has since been shown by other observers that this double fertilization occurs in many other Angiosperms, both See also:

• DICOTYLEDONS

It is possible, however, that the segregation of characters in the gametes may depend upon something far more subtle and elusive than the chromosomes or even of possible combinations of units within the chromosomes, but so far as we can see at present these are the only structures in the cell with which it can be satisfactorily associated. Boveri in fact has put forward the view that the chromosomes are elementary units which maintain an organic continuity and independent existence in the cell. The cytological evidence for this appears to be made stronger for animal than for plant cells. From numerous investigations which have been made to trace the chromosomes through the various stages of the nuclear ontogeny of plant cells, it appears that the individuality and continuity of the chromosomes can only be conceived as possible if we assume the existence of something like chromosome centres in the resting nucleus around which the chromosomes become organized for purposes of division. Rosenberg (1909) adduces evidence for 1 Strasburger (1909) states very definitely that he has observed the entrance of the male nucleus into the egg without a trace of cytoplasm. [After Webber.) tion in Zamia. the existence of chromosomes or " prochromosomes " in resting nuclei in a large number of plants, but most observers consider that the chromosomes during the resting stage become completely resolved into a nuclear network in which no trace of the original chromosomes can be seen. Special Cell-Modifications for the Reception of Stimuli.—In studying the physiology of movement in plants certain modifications of cell-structure have been observed which appear to have been developed for the reception of the stimuli by which the response to light, gravity and contact are brought about. Our knowledge of these structures is due mainly to Haberlandt. Organs which See also:

• RESPOND

The response to the action of light in diatropic leaves is, according to Haberlandt, due to the presence of epidermal cells which are shaped like a See also:

• LENS
• LENS (from Lat. lens, lentil, on account of the similarity of the form of a lens to that of a lentil seed)

The Nuclei of the Lower Plants.—It is only in comparatively recent times that it has been possible to determine with any degree of certainty that the minute deeply stainable bodies described more especially by Schmitz (1879) in many Algae and Fungi could be regarded as true nuclei. The researches of the last twenty years have shown that the structure of the nucleus and the phenomena of nuclear division in these lower forms conforms in all essential details to those in the higher plants. Thus in the Basidiomycetes (fig. 7) the nuclei possess all the structures found in the higher plants, nuclear membrane, chromatin network and nucleolus (fig. 7, B), and in the processof division, chromosomes, nuclear spindle and centrosomes are to be seen (fig. 7, C–G). The investigations of Dangeard, Harper, Blackman, See also:

• MISS

D, Nuclear cavity with chromosomes. E, Chromosomes on the spindle. F, Separation of the chromosomes into two groups. G, Chromosomes grouped at opposite ends of the spindle to form the daughter nuclei. A C In the Cyanophyceae the contents of the cell are differentiated into a central colourless region, and a peripheral layer containing the chlorophyll and other colouring matters together with granules of a reserve substance called cyanophycin. Chromatin is contained in the central part together with granules known as volutin, the function of which is unknown. The central body probably plays the part of a nucleus and some observers consider that it has the characters of a typical nucleus with mitotic division. But this is very doubtful. The central body seems to consist merely of a spongy mass of slightly stainable substance, more or less impregnated with chromatin, which divides by constriction. At a certain stage in the division figures are produced resembling a mitotic phase (fig. 8, I), which are not, in A B (From Prot. See also:

• ROY, WILLIAM (1726-1790)

See also:

• SOC

Cases of complete fusion occur in the formation of laticiferous vessels, and in the spiral, annular and reticulate vessels of the xylem. Incomplete fusion occurs in See also:

• SIEVE (O.E. sife, older sibi, cf. Dutch zeef, Ger. Sieb; from the subst. comes O.E. siftan, to sift)

The nuclei of the original cells persist in the protoplasmic membrane. The rows of cells from which the laticiferous vessels are formed can be distinguished in many cases in the young embryo while still in the dry See also:

• SEED (from the root seen in Lat. serere, to sow)

The mode of formation of the sieve plate is not certainly known; but from the fact that delicate connecting threads of protoplasm are present between the cells from their first development it is probable that it is a special case of the normal protoplasmic continuity, the sieve pores being produced by a secondary enlargement of the minute openings through which these delicate strands pass. According to Lecointe, the young wall consists partly of cellulose and partly of a substance which is not cellulose, the latter existing in the form of slight depressions, which See also:

• MARK
• MARK, CARL (1858– )
• MARK, GOSPEL OF ST
• MARK, ST

9, B), or "wall protoplasm of cells of Tamus communis and eiido threads " which are present See also:

• PERM