190 . STRUCTURAL BOTANY 



to high-water mark. The plants are thus only under 

 water for a comparatively short time, not more than 

 a quarter of the day, and are able to bear a temporary 

 state of drought without injury. 



1. STRUCTURE 



Pelvetia canaliculata is usually found in abundance 

 on any rocky shore, forming a well-defined band, just 

 below the highest level reached by ordinary tides. The 

 plants are only a few inches high (smaller than most of 

 their relatives), and have a forked, flattened thallus 

 attached to the rocks by a rounded disc (see Fig. 81, A). 

 The thallus shows a conspicuous groove or furrow along 

 one side, to which the species owes its name. In 

 addition to the regular dichotomous branches, adven- 

 titious shoots may arise on any part of the thallus. 



When the plant is in fructification, which happens 

 chiefly in the late summer and autumn, the ends of 

 some of the branches become enlarged and studded with 

 wart-like projections, each of which has a minute pore at 

 the top (see Fig. 81, A, r). The swollen ends of the 

 branches are called the receptacles ; the wart-like bodies 

 mark the position of the conceptacles, which are cavities 

 in the tissue, containing the organs of reproduction (see 

 Figs. 81, B, and 82). 



Pelvetia, though one of the simplest of the Fucaceae, 

 is a very highly organised plant compared with the 

 Algae already considered, and shows a rather complex 

 anatomical structure, which we will now very briefly 

 describe, 



The external layer of tissue consists of small cells 

 with abundant plastids, giving their contents a dull 



