THE PLANT-CELL 



47 



(Hydrodictyon), the common Alga Cladophora, and the group of 

 Algae known as Siphoneae. In these cases the nucleus may divide 

 repeatedly without any division-wall being formed, so that the pro- 

 toplast remains undivided. In Hydrodictyon, the number of nuclei 

 may be several thousand. In these cases, nuclear division is of the 

 usual type (Karyokinesis), but occasionally e.g. the long cells of 

 the Characeae (Fig. 43), old cells of Tradescantia the nucleus may 

 divide by direct constriction, or fragmentation. Such direct division, 

 however, never occurs in young cells. 



Structure of the Nucleus 



The nucleus is evidently of great importance to the life of the 

 cell, and the protoplast is incapable of protracted existence if 

 deprived of the nucleus. Thus if the protoplast is divided into two 

 parts by plasm oly sis, which can be done without otherwise injuring 

 the cell, it is found that the part of the protoplast containing the 

 nucleus can develop a new wall and become a normal cell : the other 

 piece soon dies. The latter can in some cases manufacture starch in 

 the presence of light, but seems to be incapable of using it, and the 

 starch accumulates until finally the mass dies. 



In the living cell the nucleus has the form of a vesicle with a 

 clearly defined membrane, which, as already stated, is the limiting 

 layer of the cytoplasm which surrounds the nuclear cavity. The 

 latter may appear homogeneous, or more commonly shows a more 

 or less definitely granular appearance. The nucleolus is usually 

 conspicuous, and is a globular, 

 highly refractive corpuscle. 



The nuclear structures, 

 when more carefully investi- 

 gated by means of proper fix- 

 ing and staining agents, are 

 found to be very complicated. 

 The nuclear cavity is filled 

 with an apparently homo- 

 geneous fluid, the nuclear sap, 

 in which the solid elements 

 form a Complicated network FIG. 31. A, young spermatogenic cell of 

 of fibrils This is made UP Marsilia vestita in process of division ; 

 of a single, much-tangled 

 thread, which is more or less 

 fused together where the 

 strands touch, resulting in 

 the reticulated structure which can be made out in the resting 

 nucleus. In the latter the filaments composing the nuclear network 



bl 



bl, blepharoplast. (After SHAW.) B, 

 nucleus from a cell of Dictyota dichotoma, 

 showing the centrosomes, cm, with the 

 radiating kinoplasmic filaments. (After 

 MOTTIEB.) 



