124 A TEXTBOOK OF THEORETICAL BOTANY 



be produced in equal numbers. The Aglaozonia stage is comparable with 

 the asexual stage of an Ectocarpus, and the early development of the zygote 



Aglaozonia 

 Plant ' 



Zygotes 



Zoosporangium 



Male 

 Gametes 



Female 

 Gametes 



Zoospores 



Male 

 Gametangium 



Male Plant 



Female Plant 



Fig. 107. — Life-cycle oi Ctitleria. 



Female 

 Gametangium 



produces a filament comparable with an Ectocarpus plant. The flat thallus 

 is a secondary development facilitating a greater production of zoosporangia 

 (Fig. 107). 



Dictyotales 



These Algae are Phaeophyceae in which a definite alternation of genera- 

 tions occurs and in which the sporophytic and gametophytic plants are 

 morphologically indistinguishable. Sexual reproduction is oogamous, while 

 the organ of asexual reproduction is a non-motile spore termed a tetraspore, 

 so called because only four spores are produced in each sporangium. 



This is a small order with only a few genera, and we shall consider Dictyota 

 dichotnma as our example. 



Dictyota dichotoma 



This Alga is common around the coast of Britain, growing in pools between 

 tide-marks (Fig. 108). The strap-like thallus is 10 to 20 cm. long and 

 consists of rectangular cells arranged in a single layer, with a superficial 

 layer of smaller cells on each side of the thallus. Small tufts of hairs develop 

 from scattered groups of these surface cells. The thallus is attached to the 

 rock by a basal holdfast. 



The thallus branches repeatedly, each division giving rise to two equal 

 branches. Such a type of branching is termed dichotomous. Growth is 

 by means of one large apical cell on each branch, which divides vertically 

 into two equal halves when branching is about to take place. The male and 



