THE ALGJE 



117 



strip quite destitute of chromatophores, known as the neutral zone, 

 as here no movement can be detected. The cytoplasm forms a thick 

 layer within the cell-wall surrounding the single, very large vacuole. 

 The chromatophores are embedded in the outer layer, which shows 

 no movement ; but below this a thick layer of soft, very granular 

 protoplasm is seen to be in active rotating movement, the whole 

 mass moving in a single current. Numerous large nuclei, as well 

 as other protoplasmic bodies, some of peculiar form, like little 

 spiny balls, are carried along in the protoplasmic current. 



FIG. 89. A, /?, Vaucheria geminata, showing the development of the sexual 

 organs. C, young oogonium of V. clavata; the nuclei are shown black. D, an 

 open oogonium, with a single nucleus, n; sp, two spermatozoids. E, conjugation 

 of the sexual nuclei, more highly magnified. (C-E, after OLTMANNS.) 



Nuclei. In the young cells there is a single large nucleus, of the 

 ordinary form, which divides by mitosis when new cells are formed. 

 In the large cells, however; the nucleus undergoes repeated direct 

 division, or fragmentation, resulting in numerous large nuclei, often 

 of very irregular form (Fig. 90, B). 



Apical Growth. All of the organs of the plant grow from an 

 apical cell, which, in the larger shoots, has the form of a hemisphere 

 (Fig. 90, B, v). From its base segments are cut off, in regular suc- 

 cession, by transverse walls. Each disk-shaped segment divides 

 quickly by a second transverse wall, which is usually convex 



