SECT. 1 



CRYPTOGAMS 



359 



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

 aplanospores \vhich suiTound themselves with a wall aud grow to the full size. 

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



Scenedesmus ("-), which is widely spread 

 in fresh water, and connects on to 

 Chlorella. The commonest forui, Sc. 

 ac'Utus, has spindle-shaped cells, while 

 the colonies of Sc. candcdus are distin- 

 guished by four long horn -like pro- 

 longations of the cell wall (Fig. 'J78). 

 In reproduction each cell divides in 

 the direction of its length into four 

 daughter-cells, which on escaping from 

 the parent cell form a new colony. 

 More complicated cell colonies are met 



33 \ 



Fig. 277. — Chlorella vulgaris, i, Cell ; ^, .3, division 

 into four aplanospores ; U, 5, division into eight 

 aplanospores. (After Grintzesco.) 



with in Fcdiastrum (^•^) (Fig. 279), in 



which each cell -family forms a free- 

 swimming plate, comjiosed internally 

 of polygonal cells, while on the margin it consists of cells more or less acutely 

 creuated. The formation of asexual swarm-spores is effected in Pcdiastnun by 

 the division of the contents of a cell into a number (in the case of the species 

 illustrated, P. granulatum, into 16) of naked swarm-spores, each with two cilia. 

 The swarm-spores, on escaping through the ruptured cell wall (Fig. 279, A, h), are 

 enclosed in a common envelope. After 

 first moving vigorously about within this 

 envelope, they eventually collect together 

 and form a new cell-family. Pcdiastrum 

 also possesses a sexual mode of reproduc- 

 tion. The gametes are all of equal size, 



Fig. 278. — A, Scenedesmus acutus. B, the same, 

 undergoing division ; C, Scenedesmus caudatus. 

 (After Senn, x 1000.) 



Fifi. 279. — Fediastnim (jraiiulatum. A, An old 

 cell-family : a, cells containing spores ; 6, 

 spores in process of extrusion (the other 

 cells have already discharged their spores) ; 

 B, cell-family shortly after extrusion of the 

 .spores ; C, cell-family 4^ hours later. (After 

 Al. Braun, X 300.) 



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 paii's to form zygotes. The further development of the zygotes into cell- 

 families is not yet fully known. 



The life-history of the Water-net {Hydrudictyon ntriculatum (^' ■*"') is essentially 

 similar. It is one of the most beautiful of the free-floating, fresh-water Algae, the 



2 A 2 



