132 



L. digitata. 



A TEXTBOOK OF THEORETICAL BOTANY 



There are periodic openings from these ducts to the exterior 

 and they are hned with isolated groups of secretory 

 cells. They arise schizogenously between cells of 

 the surface layer and deepen and extend with 

 growth, becoming connected with each other into 

 a continuous network. 



The structure of the attaching organ or 

 holdfast is markedly different from the rest of 

 the thallus. Growth is localized in the apices 

 of the branches which spread out in contact 

 with the rock. It differs anatomically by the 

 absence of a medulla and of trumpet hyphae. 



Asexual Reproduction 



The asexual reproductive organs are developed 

 in widely extended sori, which may cover the 

 greater part of the surface on both sides of the 

 lamina (Fig. 121). The zoosporangia arise from 

 the superficial cells in the following way. Each 

 cell divides into two, forming a basal cell and a 

 terminal cell. The terminal cell is at first assimi- 

 latory, but later enlarges considerably, becomes 

 club shaped and is invested at the top by a 

 mucilaginous cap. This body becomes a para- 

 physis, and the caps of all the paraphyses adhere 

 and serve to keep them together. Meanwhile the 

 basal cell enlarges laterally and from its outer 

 ends cuts oft' two cells, one on each side of the 



terminal cell. Each enlarges considerably, becomes oval and forms a 



sporangium. These sporangia thus 



come to lie between the paraphyses, 



and the whole is covered by the 



mucilage derived from the latter. In- 

 side the zoosporangium thirty-two 



zoospores are differentiated, and are 



finally liberated through the apex of 



the sporangium. According to most 



workers these zoospores are all of the 



same size, though in one or two cases 



zoospores of different sizes have been 



described. 



The zoospores are almost pear- 

 shaped bodies with two long, laterallv 

 1 1 n 1, J , " Fig. iiq. — Lamina) m sacchnnna. On left 



placed tlagella, and each may possess a the surface view of the sieve plate in a 



tiny eye spot. They are actively motile trumpet hypha. On right an old 



1 . ^^1 J 1 " • trumpet hvpha with callus accumulated 



but soon settle down and germmate. on the sieve plate. {After Sykes.) 



Fig. 1x8. — Laminaria digitata. 

 Trumpet hypha in medulla 

 showing the enlarged ends 

 of two hyphae in contact 

 and the trans\erse ribbing 

 of the hyphae. 



