THE ALGAE 



"5 



2x 



conjugation like zoospores. The products of the unilocular sporangia are, 

 however, monoploid. Most observers agree that they too germinate without 

 fusion and regard them as true zoospores. It has been claimed, however, 

 that they may fuse in pairs, producing diploid plants, but the actual develop- 

 ment of such apparent conjugations has never been observed. If it occurs 

 we have here a complete inversion of the normal reproductive order. 



The occurrence of con- 

 jugation between the motile 2x 

 cells produced in sporangia 

 has been interpreted as im- 

 plying, in these cases, the 

 complete suppression of the 

 gametophyte stage, the pro- 

 ducts of the sporangia be- 

 having directly as gametes 

 instead of germinating to 

 produce another vegetative 

 generation. This special case 

 in Ectocarptis would therefore 

 be comparable to that in the 

 normal life-history of Fiiciis 

 (Fig. 96). 



Cutleriales 



The Cutleriales are Phae- 

 ophyceae in which there is an 

 alternation of generations be- 

 tween a large upright gamet- 

 ophyte and a small prostrate 

 sporophyte. The gametangia 

 are borne on short filaments 

 arising from the surface layer y\g 

 of separate gametophyte 

 plants, and the number of 

 gamete cells produced in 

 the male gametangium is 

 considerably greater than those 

 in the female. It is only on the structure of the sex organs that the sexes 

 can be distinguished. The asexual plants give rise to zoosporangia, from 

 which a number of zoospores are liberated. Growth of the thallus is peculiar. 

 The apices of the branches are composed of clusters of uniseriate filaments. 

 These filaments coalesce below to form a solid tissue. At or near the base 

 of each filament is a zone of actively dividing cells, which cut off above 

 more cells to the filaments, while at the same time contributing below to the 

 solid tissue of the thallus. Such a type of intercalary growth is sometimes 

 referred to as trichothallic. 



B 



96. — Ectocarpiis. Variations of the life-history. 

 A, Diplobiontic type with both diploid and 

 monoploid plants. Gametangia produce dip- 

 loid swarmers which germinate without con- 

 jugation. B, Haplobiontic type. All plants are 

 diploid. The zoosporangia produce monoploid 

 swarmers which conjugate in pairs. 



