Page:The New International Encyclopædia 1st ed. v. 04.djvu/469

* CELL. 401 CELL. rived from the pi-oto|>lasin. As growth pro- ceed!^, the original wall of cellulose may become inoditied, the various changes heing called ligni- fication, suberization, etc. In spores the outer portion of the wall usually develops spines, fur- rows, and various sculpt uriiigs. that are sulli- cientl.v constant to be of value in classification. The internal modifications of the cell wall are no less numerous and striking. Sometimes there is a uniform thickening, but more conunonly the thickening is irregular and results in the ap- pearance of striations, spirals, rings, reticula- tions, and dilt'erent forms of pits. Contexts. The most important contents of the cell are the nucleus and the c.vtoplasm, but besides these various other things nia.v l>e pres- ent, such as plastids, starch-grains, crvstals, oil- drops, sap. etc. Plastids are dift'erentiated por- tions of the cytoplasm, and it has been believed by many that the.v are iieniianent orgsins of the cell, passed on from one cell generation to an- other, and it is luidoubtcdly trne that they divide and may persist throughout the greater part of the life history of a plant: but whether the,v are present in the initial cell of the plant or not is a serious question, ami if not, it must be admitted that the,v ma.v he formetl de iiavo from the cytoplasm. Colorless plastids are called ieucoplasts,' and colored ones 'chromato- phores.' If a chromatophore contains cliloro- ph.vll (the green pigment) it is called a Vhloro- plast.' but if it contains any other coloring matter it is a 'chromo))last.' The function of the leucoplast is to produce starch, but if light con- ditions are favorable the leucoplast may be con- verted into a chloroplast. Chloroplasts vary greatlv in form, in size, and in the number found in a single cell. In the higher plants the.v are small flattened or discoid bodies, and many are usuall.v found in a single cell. However, in Anthoceros (a liverwort) and Selaginella (little club-mosses), there is only one in a cell. The greatest variety of forms is found among the seaweeds (Alga?), where there are not onlv the small discoid forms, but also flattened plates, coiled bands, etc. The chloroplasts, like the Ieu- coplasts. produce starch, but the,v are also able to manufacture carbohydrates in the presence of light (photosynthesis, q.v. ). Starch-t^rains var^ in form and size. While the size may vary greatl.v with age. the form is usually character- istic of a given plant. The starch formed by the chloroplasts is in the form of very small gran- ules, which do not become large grains, but dis- appear when light is remove<l, and are probablv carried to other portions of the plant. The ordinar,v starch-grains are developed in the pro- toplasm. Crystals composed of calcium oxalate are widel.v distributed. In form the.v are pris- matic, cubical, octohedral, acicular. etc. Those which are needlelike in fonn are called 'raphides.' C.vstoliths composed of calcium carbonate are much less common. CYTOPi,A.sii AND XtCLEUS. These are the liv- ing portions of the cell. So far as known, there is no difference in the structure of the c.vtoplasm of animal and plant cells. The nutritive por- tion of the c.vtoplasm has been termed the 'trophoplasm,' and the thread-like portions which are particularlv conierned in cell-division and other phenomena of motion are called 'kinoplasm.' The bounding ('limiting') laver of the c'l'toplasm is called the "Hautschicht.' While cells are young, the cytoplasm tills the entire space be- tween the nucleus and the cell-wall, but as the cells increase in size vacuoles (apparentlv empl.v spaces) api)ear, and the c.vtoplasm merely forms a wall laver connected with the nucleus by con- stantl.v shifting strands. XrcLEis. The muletis is an exceedingl.y com- plex structure. An umloubtcd nucleus has been demonstrated in all groups of plants except in blue-green Alga' (C.vano])hycea;) and Bacteria. From the seed-plants down to the Alga; a single nucleus in a cell is the rule, but there are man.v exceptions. The absolute size of nuclei is ex- ceedinglv variable. As a rule, it mav be said that large cells have large nuclei. The nuclei of reproductive cells are gencrall.v larger than those of the vegetative cells of the same plant. The nuclei of tiie lilies, orchids, and conifers are usuall.v large, while those of mosses and dico- t.yIedons are small. For example, the nucleus of the egg of the common Austrian pine {t'inus laricio) can be seen with the naked eye, l>eing 0.006 inch in diameter; while the nuclei of some of the fungi are scarcelv 0.00008 inch in diameter. The nuclei of the lilies, which are seldom more than 0.002 inch in diameter, may be regarded as uuusuall.v large, and they have been studied more than anv other plant nuclei, FoRH OF XucLErs. The nucleus in .voting cells is usuall.v spherical, but as cells grow older its form mav change in innumerable wavs. When cells become elongated, as in the develop- ment of vascular tissue, the nuclei ma.v also elongate, the length becoming a hundred times as great as the breadth. In the mucilaginous sap of some amarvUises nuclei have been ob- ser'ed in which the length had become more than iive thousand times as great as the breadth. In the endosperm of Indian corn the nuclei acquire remarkable reticular forms, while in some of the stonecrops (Sedion) they liecome variouslv lobed. In some pollen-grains one of the nuclei becomes so amoeboid in form that it is easv to believe that it ma.v be capable of ama'boid movements; and in one of the slime-molds such movements have actuallv been observed. Division of Xccleis. It is accepted that a nucleus never arises de novo, but onl.v bv the division of a preexisting nucleus. The principal sti"uctures concerned in division are the chromo- somes, the linin, the spindle (achromatic figure), and the centrosome. The last structure, how- ever, is not so constantlv present as in animals, and many believe that it does not exist at all in the fern-plants (pteridophvtes) and seed- plants (spermatoph-tes) . In the cells which produce pollen-grains the spindle first apjiears as a weft of fibres surrounding the nucleus, which become grouped into cones forming a mul- tipolar spindle, that graduallv becomes bipolar. In ordinar.v vegetative cells the spindle is bi- polar from the start, making its a|>pearance as a pair of dome-shaped caps at opposite poles of the nucleus. The chromatin is generallv re- garded as the most imijortant part of the nu- cleus, as it is believed to be the morphological basis of heredity. During division the chromatin assumes the form of a narrow band which divides into a definite number of more or less elongated pieces, the chromosomes. The number of chromo- somes is constant in a given species, and the number in the sexless generation (sporophyte) is always double the number found in the sexual