220 SEXUAL REPRODUCTION (ECTOCARPUS) 



proved to be detrimental to the organism. On the other hand, 

 there are quite a number of Algae in which a sexual process is 

 unknown, as in the whole group of the Cyanophyceae and in 

 many Desmids. Apart from these, however, most Algae re- 

 produce sexually at some time or other in the course of the year, 

 most commonly during the spring. In many cases sexual 

 fusion results, as in Ulothrix, in the production of resistant 

 spores and, in contrast to the asexual method, is frequently 

 associated with the onset of conditions adverse to the plant's 

 growth and nutrition. 



The fusing gametes are outwardly alike in Ulothrix and 

 Cladophora, as well as in all species of Chlamydomonas, excepting 

 C. monadina. The dissimilarity in size and behaviour of the 

 sexual cells seen in the last-named species (cf . p. 185) is paralleled, 

 or even more emphasised, in the higher Algae. A relatively 

 simple instance is afforded by Ectocarpus, where the gametes 

 are produced in special gametangia, occupying the same lateral 

 position as the sporangia, but differing in their more elongated 

 shape and in being divided into numerous compartments in 

 each of which a single gamete is formed (Fig. 119, a, g.). The 

 sexual cells are smaller, but otherwise resemble the zoospores. 

 Despite their structural uniformity, some gametes are relatively 

 sluggish, and, after a brief period of movement, become attached 

 to any suitable substratum by a disc-like expansion at the end 

 of their forward cilium, whilst others move actively and for a 

 much longer time. The latter ultimately collect in groups around 

 the others and, sooner or later, an active gamete fuses with a 

 resting one,(Fig. 119, h and i). There is thus a marked difference 

 in behaviour between the two fusing cells, but the differentiation 

 into active males and passive females is here purely physiological, 

 although in a few species of Ectocarpus the two gametes differ in 

 size and other respects. 



The distinction between the two sexes is much more marked in 

 forms like (Edogonium, Vaucheria, and Fucus, where one sexual 

 cell (the female or egg) is large, motionless, and provided with 

 plentiful food-material, whilst the other (the male or sperma- 

 tozoid) is small, actively motile, and possessed of very scanty 

 cytoplasm. The two kinds of gametes ar usually formed in 

 special sexual organs differentiated from ordinary vegetative 



