ATOMS, MOLECULES, AND CELLS AS FACTORS 141 



legists refer all the changes which occur in reproduction, in the embryo, and in the tissues of plants and animals, 

 directly or indirectly, to the cell. Professor Virchow says, " Omnis cellula e cellula." 



Those who advocate this view maintain that all the tissues are developed from cells, and that the first changes 

 in the embryo are inaugurated by cell structures. They assert, consequently, that either the vitelUne membrane 

 sends in processes to divide the yolk mechanically, or that the germinal vesicle separates into two ; the two parts 

 attracting the molecules of the yolk around them. Cells, it is claimed, take the lead in reproduction,a bsorption, 

 assimilation, nutrition, growth, secretion, excretion, &o. 



The nature of the cell in cellular plants, the function discharged by it in plants generally, and the manner in 

 which it divides and multiplies to form tissues and new individuals, are shown at Plate lix.. Figs. 1 to 4, and 

 Plate iii. The unicellular animals, and the behaviour of cells in animals, are very fully illustrated in various 

 parts of the work. 



PLATE LIX 



Plate lix. deals with cells, cell contents, cellular plants, spores, and the lower plant forms. The cellular animals 

 and lower animal forms are given at Plates Ixi. to Ixiv. inclusive. 



Fig. 1. — A. Cells ot the green-snow plant (Protococciis viridis), when fully developed. They contain germs or cellules which when 

 discharged produce new plants. The cells perform nutritive and reproductive functions. 



B. Cells of the yea.st plant (Toriila cerevisix) in different stages of growth, a, Cell in an early state ; h, cell with nucleus ; c, cell 

 with nucleoli. This cell plant, like the green-snow plant, performs nutritive and reproductive functions. 



C. Cells of the red-snow plant (Protococcus niwlis), in different stages of development, a, Cell in -the young state ; h, cell fully 

 formed, containing cellules ready to be discharged, and to form new plants ; c, cell with contents discharged. Each cell performs the 

 functions of nutrition and reproduction. 



D. A species of mould-fungus (Botrytis), consisting of a mycelium, bearing a separate cellular stalk, which branches at the apex ; 

 each division bearing a rounded spore. 



E. Elongated club-shaped cell of Vaucheria ovoidea, discharging a cellular spore which has been formed in its interior. 



F. Spore with numerous cilia surrounding it (a) ; b, spore with numerous cilia at one end ; t, small cell or spore with two 

 vibratile filaments. 



G. Two filaments of a cellular plant (Zygnema), united by means of tubes. The plant is formed by a series of cells arranged end 

 to end in a single row. Note — The cells contain S-shaped contents (a) ; spiral contents (b) ; and spiral figure-of-8 contents (c). 



H. Thallus of the common bladder sea-weed (Fucus vesiculosus), consisting of numerous cells united ; some {a, a) being nutritive, 

 and others (b, b) reproductive. 



I. Elaters, or spiral fibres, with spores or reproductive cells (a, a) from liverwort (Marchantia). The fibres are elastic, and serve 

 to scatter the spores (Balfour). The scattering of the spores by elaters is a designed provision of nature. 



Fig. 2. — A, B, C, D. Longitudinal section of a young, half-grown cell from the cortical parenchyma. The peduncle of Gephalaria 

 leucantha, treated (B) by a four per cent, solution of potassium nitrate, (C) by a six per cent, solution, and (D) by a ten per cent, 

 solution, a, Cell wall ; b, protoplasmic lining of cell wall ; c, cell nucleus ; d, chlorophyll-granules ; e, cell sap. The lettering given 

 for A applies equally to B, C, and D. /, Salt solution which has pas.sed through the cell wall (De Vries). 



Fig. 3.— Longitudinal section of the middle cell of the hair of a gourd (from the calyx of the young flower-bud), n. Cell wall ; 

 b, vacuolated mass enclosing the nucleus of the cell; c, branching filaments of protoplasm in active movement and carrying 

 chlorophyll corpuscles (containing starch in their substance) ; d, a crystal being hurried along (after Sachs). 



Fig. 4. — A. Tetraphis pellucida, a plant bearing gemmje (natural size). 



B. The same magnified ; at top of figure is the cup in which the gemmae collect. 



C. Longitudinal section through the apex of B, the gemmae being seen in various stages of development. 



D. Mature gemma, x 550, consisting at the margin of one, and in the centre of several layers of cells (after Sachs). 



E. Development of Pandorina morum : a swarming family of same. 



F. a similar family developed into sixteen daughter-families. 



G. A sexual family, the individual cells of which are escaping from their gelatinous investment. 

 H, I. Conjugation of pairs of s warmers. 



J. Zygote which has just been completed. 



K. A fully grown zygote. 



L. Transformation of the contents of a zygote into a large swarm-cell. 



M. The same after being set free. 



N. A young family developed from the latter (after Pringsheim). 



PLATE LX 



Plate Ix. deals with the pollen and cells of plants, and with transverse, longitudinal, and other sections of 

 shrubs and trees. The transverse sections show the concentric arrangements of the cells and woody fibres in 

 certain cases. The radiating arrangements are seen at Plates xxii., xxiii., and xxiv., pp. 39-41. Various kinds of 

 pollen are given at Plate Ixv. p. 164. 



Fig. 1. A. Funhia cordata. Transverse section of a young pollen sac, before the isolation of the mother cell, a a'. Mother cells ; 

 b, epithelium (tapetum) clothing the loculus; c, wall of the pollen sac. 



B. Loculns after isolation of the mother cells (a a') ;*. I'eraai"^ of tapetum, X 550. .,.,,, , t -vt ^ .^ 



C. Funkia omta Development of pollen, x 550. The development proceeds as indicated by the numerals. In No. / the 



