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 

 (Edogonium ; 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 Chroolepus 

 does. Ulothrix itself, which is 

 commonly found attached to 

 stones washed by a quickly 

 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 (13). 

 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 two 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 



Fie. 330. 



Ulothrix zonata. A, young filament with rhizoid r 

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

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

 E -= gametes. F = conjugation. G= zygote. H = zygote. 





