DIV. I 



THALLOPHYTA 



401 



formed cell walls. (After BEYERINCK, from 

 OLTMANNS' Algae.) 



often live symbiotically in the protoplasts of lower animals (Infusoriae, Hydra, 

 Spongilla, Planariae) ; it is multiplied only by division of the cells into 2, 4, or 8 

 aplanospores which surround themselves with walls and grow to the full size. 



The simplest type of cell colony, consisting of four cells, is found in the genus 

 Scenedcsmus, which is widely spread 

 in fresh water, and connects on to 

 Chlorella. The commonest form, Sc. 

 acutus, has spindle-shaped cells, while 

 the colonies of Sc. caudatus are dis- 

 tinguished by four long horn-like pro- 

 longations of the cell wall (Fig. 337). 

 In reproduction each cell divides in 



the direction of its length into four ^ ^HHH^ ~ Wty 9 



daughter cells, which on escaping from 

 the parent cell form a new colony. More FlG ^^^chlorococcum(Chlorosphaera) limicola. 1, 

 complicated cell colonies are met with Vegetative cell and cell divided into 8 zoospores; 

 in Pediastrum (Fig. 338), in which f, free zoospores ; 3, zoospores after they have 

 each cell-family forms a free-floating 

 plate, composed internally of polygonal 

 cells, while on the margin it consists of cells more or less acutely crenated. The 

 formation of asexual swarm-spores is effected in Pediastrum by the division of 

 the contents of a cell into a number (in the case of the species illustrated, P. granu- 

 latum, into 16) of naked swarm-spores, each with two cilia. The swarm-spores, 

 on escaping through the ruptured cell wall (Fig. 338 A, 6), are enclosed in a 

 common envelope. After first moving vigorously about within this envelope, they 



eventually collect together and form a 

 new cell-family. Pediastrum also pos- 

 sesses a sexual mode of reproduction. 

 The gametes are all of equal size, and, 

 except that they are smaller and are 

 produced in greater numbers, are 

 similar to the swarm - spores. They 

 move freely about in the water, and 

 in conjugating fuse in pairs to form 

 zygotes. The further development of 

 the zygotes into cell-families is not yet 

 fully known. 



Fio. 336. Chlorella i-ulgaris. l, Cell ; , 3, division The life-history of the Water-net 

 into four aplanospores; *, 5, division into eight (Hydrodictyon utriculatuni) (***) is 

 aplanospores. (After GRINTZESCO.) , . ,, . ., T , . ,. ,, 



essentially similar. It is one of the 



most beautiful of the free-floating, fresh-water Algae, the hollow cylindrical colonies 

 being formed of elongated cells united together to form a many-meshed net. 



The Protococcales like the Volvocales can be derived from .the Flagellata. In 

 contrast to the latter group the non-motile, non-ciliated condition of the cells has 

 become prevalent as it has throughout the higher Algae. In some genera of the 

 Protococcales even the spores do not develop cilia, although, as a rule, the repro- 

 ductive cells of the Algae tend to retain the Flagellate character. The loss of 

 motility is accompanied by a more complex external form of the cells. 



Order 3. Ulotrichales 



The Ulotrichales exhibit, as compared with the unicellular green Algae, an 

 advance in the external segmentation of the thallus. It is always multicellular, 



2D 



