392 



BOTANY OF THE LIVING PLANT 



The Ulothricales are relatively advanced in development of their 

 plant-body, owing to repeated cell-division, the products of which 

 remain associated together to form filaments, as in Ulothrix or 

 CEdogonium ; or the filaments may be branched, as in Bulbochaete ; 

 or flattened expansions may be formed, as in Ulva or Enteromorpha. 



The plants inhabit salt or 

 fresh water, or may even grow- 

 in moist air, as Chroolepiis 

 does. Ulothrix itself, which is 

 ■ commonly found attached to 

 stones washed by a qxiickly 

 running stream, serves as a 

 simple example. Each un- 

 branched filament consists of 

 a series of discoid cells, each 

 with a zonal chloroplast, and 

 it is attached by a basal 

 rhizoid (Fig. 330, A). Its pro- 

 pagation though varied is 

 rudimentary like its vegeta- 

 tive structure. Zoospores may 

 be produced either singly 

 from a cell or by division of 

 its contents, which escape 

 through an opening of the 

 cell-wall into the water [B). 

 According to the number of 

 the divisions the zoospores 

 may differ in size. Each has 

 four cilia attached to the 

 narrower end of its pear- 

 shaped body [C). After a 

 period of movement they 

 settle, form a cell-wall, and 

 affix themselves to some solid substratum : growing out transversely 

 to their former axis and dividing, each may form a new filament. The. 

 gametes are also produced in a similar way, but the divisions are more 

 numerous, their size smaller, and they bear only tiz'o cilia {E). The 

 gametes, which are all alike in size and form, escape from cells : if those 

 from different filaments meet they coalesce in pairs, the result being 

 a four-ciliate zygote, which soon loses its cilia, settles, and forms a 



Fig. 330. 

 Ulothrix zonata. A, young filament with rhizoid r 

 ( A 300). B, portion of filament with escaping zoospores. 

 C, single zoospore. D, formation and escape of gametes. 

 £=gametes. F = conjugation. G=zygote. // = zygote. 

 / = zygote after period of rest. K = zygote after division 

 into zoospores. (After Dodel-Port.) {B-K x 482.) 

 (From Strasburger.) 



