I20 A TEXTBOOK OF THEORETICAL BOTANY 



seen with the naked eye. Meanwhile the columnar tissue has ceased to 

 grow, though the prostrate thallus continues to increase by marginal growth. 



The Mature Sporophyte {Aglaozonia reptans) 



For a long time the sporophyte oiCutleria was thought to be an independent 

 seaweed and was known as Aglaozom'a reptans (Fig. 102). It possesses a 



Thallus lobe 



Fig. 102. — Aglaozonia reptans. Thalli growing attached 

 to rock. Natural size. 



creeping habit, and grows on rocks and sea urchins' shells to which it becomes 

 attached bv means of rhizoids, developed from the superficial layer of its 

 lower surface. It may reach a diameter of two or three inches (Fig. 103). 

 It was not until 1898 that it was shown that Aglaozonia reptans was really 

 the sporophyte of Cutleria niultifida. 



A section through the thallus reveals one or two layers of small cells 

 on the upper and lower surfaces, and in the middle two or three layers of very 

 large parenchyma cells. From any superficial cell of the lower surface a 

 rhizoidal holdfast may arise. 



The Zoosporangium 



The zoosporangia are produced on the upper surface of the thallus and 

 are formed in patches, each composed of hundreds or thousands of zoo- 

 sporangia, which are packed close together in a palisade arrangement (Fig. 

 104). Each zoosporangium arises from a superficial cell of the thallus, which 

 elongates slightly and divides to give a small basal stalk cell and an upper 

 cell, which becomes the zoosporangium. This latter cell enlarges and 

 elongates until finally it is about three times as long as it is wide, with a 

 somewhat swollen top. The nucleus of the zoosporangium now divides 

 by reduction division, which is followed by further mitotic divisions, until 



