126 A TEXTBOOK OF THEORETICAL BOTANY 



The term tetraspore, which is appHed to them, is used also for the 

 asexual spores in the Rhodophyceae (see p. 158) which are also formed in 

 fours, but the number is not constant in the Dictyotaceae, some genera 

 producing eight in each sporangium. The tetrasporangia are, of course, 

 equivalent to the sporangia of Ectocarpus. 



On germination a tetraspore produces a fresh Dictyota plant, resembling 

 in appearance the parent from which it was developed, but bearing gametes. 



Sexual Reproduction 



The sex organs are antheridia and oogonia. They are arranged in groups 

 or sori on the surface of the thallus on separate plants. Each antheridium 

 arises from a superficial cell of the thallus (Fig. no). This cuts off a lower 



Antheridium 

 Sterile cell 



Basal cell 



Fig. 1 10. — Dictyota dichotoma. Section of gametophytic thallus with 

 sorus of antheridia. Note the sterile marginal cells. 



Stalk cell and an upper or antheridial cell which enlarges and divides, like a 

 gametangium of Ectocarpus, into a large number of parts, so that, at maturity, 

 some 1,500 antherozoids are liberated. As there are between 100 and 300 

 antheridia in each sorus, and as the number of sori on a full-grown plant 

 may be well over 3,000, there is a fortnightly production of something like 

 500 million antherozoids from each mature plant, which means that there are 

 on the average about 6,000 male gametes to each female. These antherozoids 

 are pear-shaped and very small, and each is provided with two lateral flagella, 

 the one which is directed backwards being much reduced in length. Each 

 antheridial sorus is surrounded by two or three rings of sterile cells, called 

 the paraphyses. 



The oogonium also develops from a superficial cell of the female thallus 

 (Fig. III). This cell divides into two, the lower forming a stalk cell, and the 

 upper the oogonium. Inside the oogonium a single oosphere is formed, 

 which is liberated as a naked mass of protoplasm by the breakdown of the 



