172 THE ALGAE 



Eudesme type by further segmentation of descending hyphae to 

 form a pseudo-parenchyma. The epidermal layer is ultimately 

 clothed with sporangia, paraphyses and deciduous mucilage hairs, 

 whilst the central cells become much elongated and support the 

 filaments that go to form the diaphragm. The zoospores on ger- 

 mination give rise to small filamentous gametophytes, the male 

 plants being composed of small cells, each with two to four chloro- 

 plasts, and the female of larger cells with more numerous chloro- 

 plasts. The gametangia are borne laterally or terminally on short 

 branches, but the plants do not become fertile for at least three 

 months after their formation and they usually require six months. 

 After fertihzation the oospore remains attached to the wall of the 

 oogonium. The macroscopic plant is an annual. 



* Laminariaceae : Laminaria (a thin plate). Figs. 97-100 



This genus has a very wide distribution in the waters of the 

 north temperate and Arctic zones, and it is commonly studied 

 because its morphology is characteristic of the group as a whole 

 with the exception of Chorda. Moreover it was among the first 

 genera in which the existence of a dwarf gametophyte was estab- 

 lished, thus leading to a new orientation of ideas in the classification 

 of the Phaeophyceae. The expanded lamina has no mid-rib and is 

 borne on a stipe that arises from a basal holdfast which can vary 

 greatly in form. The simplest transition area from stipe to lamina 

 is quite plain, but one may also find folds, ribs or callosities in that 

 position, which is also the region of intercalary growth. Laminaria 

 sinclairii has been studied by Setchell (1905) in some detail in 

 connexion with regeneration, a feature common throughout the 

 genus. Three types of growth can be recognized, all of them con- 

 fined to the stipe, whilst it is also possible to find all three processes 

 taking place in one individual : 



(i) The ordinary growth and extension of the blade during the 

 growing season. This hardly merits the description of con- 

 tinuous physiological regeneration given to it by Setchell 

 unless the concept of regeneration is to have a wider sig- 

 nificance. 

 (2) Periodic physiological regeneration which represents the 

 annual process whereby the new blade is formed. The transi- 

 tion area bulges, due to new growth in the medulla and inner 

 cortex, and then ruptures from the pressure, thus leaving the 



