404 



BOTANY 



PART II 



dwarf male filaments thus derived from the androspores, spermatozoids are pro- 

 duced which are set free by the opening of a cap-like lid (Fig. 341 D t a). 



The genus Coleochaete ( 35 ) is also oogamous (Fig. 343). The long colourless neck of 

 the flask-shaped oogonium opens at the tip to allow of the entrance of the spermato- 



zoid. The spherical oospore increases 

 in size and becomes surrounded by 

 a single layer of pseudo-parenchy- 

 matous tissue derived from filaments 

 that spring from the stalk cell of the 

 oogonium and neighbouring cells. 

 In this way a fruit-like body is 

 formed. On germination the oospore 

 undergoes a reduction division and 

 divides into 16-32 wedge-shaped 

 cells, then breaks up and liberates 

 ^ \ \ \\ a swarm-spore from each cell. 



Order 4. Siphonocladiales 



The Algae of this order are fila- 

 mentous and usually branched ; 

 they are distinguished from the 

 Ulotrichales by their large multi- 

 nucleate cells, the chloroplasts of 

 which are either solitary, large, and 

 reticulately- formed, or appear as 

 numerous small discs. 



The genus CladopJiora, numerous 

 species of which occur in the sea 

 and in fresh water, is one of the 

 most important representatives of 

 the order. 01. glomerata (Fig. 84) 

 is one of the commonest Algae in 

 streams, often attaining the length 

 of a foot. It is attached by rhizoid- 

 like cells, and consists of branched 

 filaments with typical apical growth 

 which some other representatives 

 of the order do not show. The 

 structure of the cells is represented 

 in Figs. 7, 9, and 18. Branching 

 takes place from the upper ends of 

 the cells by the formation of a pro- 

 trusion which is cut off as the first cell of the branch. Asexual reproduction 

 is by means of biciliate zoospores (Fig. 344), which arise in numbers from the 

 upper cells of the filaments, and escape from these sporangia by a lateral opening 

 in the wall. The sexual reproduction is isogamous as in Ulothrix. 



Only in the genus Sphaeroplea has the sexual reproduction become oogamous. 

 S. annulina consists of simple filaments and occurs in fresh water. 



Many forms occur in the sea (e.g. Siphonocladus), and some have a highly 

 complicated thallus, which is always, however, formed of branched filaments ; by 

 calcareous incrustation some forms come to resemble coral. Acetabularia mediter- 



FIG. 340. Ulothrix zonata. A, Young filament with 

 rliizoid cell r (x 300) ; B, portion of filament with 

 escaping swarm-spores ; C, single swarm-spore ; D, 

 formation and escape of gametes ; E, gametes ; F, G, 

 conjugation of two gametes ; H, zygote ; J, zygote 

 after period of rest ; K, zygote after division into 

 swarm-spores. (B-K x 482. After DODEL.) 



