48 



MORPHOLOGY 



being most abundant in northern latitudes. In the North Atlantic the 

 large forms are chiefly species of the Laminaria type, the body resembling 

 a huge leathery and stalked leaf blade sometimes 9 to 10 m. long, an- 

 chored by a roodike holdfast (fig. 123). The most remarkable forms, 

 with variously arranged blades, occur on the Pacific coast, among 



them being Macrocystis 

 (giant kelp), whose huge 

 bulk sometimes extends 

 200 to 275 m. (fig. 124); 

 Nereocystis (bladder 

 kelp), with long flexible 

 stem (reaching 60 m.) 

 that swells at the end into 

 a large globular float to 

 which are attached large 

 blades (fig. 125) ; Les- 

 sonia, with a dichoto- 

 mously branching stem 

 like a tree trunk; and 

 Postelsia or sea palm, 

 whose name suggests its 

 habit (fig. 126). 



The bodies are differ- 

 entiated into blade, stipe 

 (sometimes extremely 

 long), and holdfast. The 

 holdfast often takes the 

 appearance of an exces- 

 si vely branching and 

 very tough root system, 

 but it should not be con- 

 fused with roots in either 

 structure or function. There is also often a marked differentiation 

 of the cells into distinct tissues. For example, a section of the stipe of 

 Nereocystis shows an outer pigment-bearing zone (cortex), a zone of 

 storage cells, and a pith region (medulla) of loosely woven elongated 

 cells. Although often very large and complex in form and tissues, the 

 kelps, so far as known, are very simple in their reproductive methods. 

 The gametangia occur in dense masses on certain portions of the ordinary 



Fig. 126. — Postelsia (sea palm) ; showing numerous 

 blades from the massive stipe, and the system of hold- 

 fasts. 



