REPRODUCTION OF ULOTHRIX 



215 



drought, the cells of the ordinary Ulothrix- threads, after thicken- 

 ing their walls and becoming laden with food-reserves, often 

 fall apart and form as many separate resting spores (Fig. 116, B). 

 During the greater part of its life-history Ulothrix is thus 

 sedentary like the majority of plants, but motility is definitely 

 associated with the customary methods of reproduction. At 

 such times a form is assumed resembling that of the unicellular 



FIG. 1 1 6. A and B, Ulothrix. A, Thread passing into Palmella-stagQ 

 (after Cienkowski). B, Thread showing formation of resting spores 

 (after Fritch). C and D, Hormidium (after Klebs). C, Thread under- 

 going fragmentation. D, Short length of filament. E and F, Ulva 

 lactuca (original). E, A small part of the thallus, seen from the 

 surface under the low power. F, A few cells, more highly magnified. 

 ch., chloroplast. 



organisms which are motile throughout their existence. The 

 reproductive cells in question are differentiated into asexual 

 zoospores and sexual gametes, but it will be gathered that 

 this distinction is here no very sharp one. 



The reproduction of Cladophora is essentially similar to that of 

 Ulothrix, but the zoospores and gametes are produced in large 

 numbers (Fig. 104, C), and their formation is usually restricted 

 to the cells of the finer branches. During periods that are 



