60 



BOTANY 



PART I 



multinuclear cells are by no means infrequent. In the Fungi, and in 

 the Siphoneae among the Algae, they are the rule. The whole plant is 

 thus composed either of but one single multi- 

 nuclear cell, which may be extensively branched 

 (Fig. 250), or it may consist of a large 

 number of multinuclear cells, forming together 

 one organism. Thus, after suitable treatment, 

 several nuclei may be detected in the peri- 

 pheral cytoplasm in the cells of the common 

 filamentous fresh-water Alga Cladophora glome- 

 rate (Fig. 6, p. 12) (Fig. 60). 



The nuclei of the long, multinuclear cells 

 (Fig. 61, n) of fungoid filaments, or HYPH.E, 

 and also of many Siphoneae, are characterised 

 by their diminutive size. 



The Origin of the Living Elements of 

 Protoplasm. — Every nucleus in an organism 

 owes its origin to the nucleus of the' germ 

 cell (egg or spore) ; the nuclei of the germ 

 cells are descended from the nuclei of previous 

 generations. The spontaneous formation of a 

 nucleus never takes place. In the same 

 manner, the cytoplasm of -every organism is 

 derived from the cytoplasm of the germ cell, 

 and, so far as is yet known, both centrospheres and chromatophores 

 take their origin, each only from its own kind. 



Nuclear Division. — Except in a few limited cases, nuclei reproduce 

 themselves by mitotic or indirect division. This process, often 

 referred to as karyokinesis, is somewhat complicated, but seems 

 necessary in order to effect an equal division of the substance of the 

 mother nucleus between the two new daughter nuclei. In its principal 

 features the process is similar in plants and animals. 



In vegetable cells, the threads composing the nuclear network (p. 

 56) first become thicker and correspondingly shorter (Fig. 62, 1), 

 the anastomosing connections forming the meshes are drawn in, while 

 the thread itself straightens out and becomes less entangled, and 

 in consequence more easily distinguished. At the same time the 

 amount of the chromatin increases, and this increases its capacity of 

 absorbing stains. Finally, the chromatin substance in the thread 

 becomes arranged in parallel discs {A) united by linin. The thread 

 itself then divides transversely into a definite number of segments, 

 the chromosomes (2, 3), which thereupon range themselves in a 

 plane in a special manner, and form the so-called nuclear plate 

 (3). Then, or sometimes before, the segments divide longitudinally 

 (4, B, C), and the halves thus produced separate (s) from each other 

 iu opposite directions to form the daughter nuclei. 



Fig. 61. — Portions of two ad- 

 jacent cells in a hypha from 

 the stalk of a Mushroom, 

 Agaricus pratensis. n, Nu- 

 clei ; m, pits, (x 540.) 



