662 



THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



stipe, and a flat, expanded leaf, deeply lobed like a hand. It is often met with two 

 metres in length. The leaf is of a leathery consistency, and the flattened or cylin- 

 drical stalk has a wide parenchymatous cortex and central "medulla", in which run 

 curious tubes (the "trumpet hyphse") which swell out at intervals, the swelling 

 being traversed in a transverse direction by a delicate sieve-plate. The stem 

 possesses a peripheral cambium-like zone, which adds each year a new zone of tissue 

 to the cortex. The stems, which are sometimes found much thicker than one's 



i'ig. 375.— Laminariacese, with perforated fronds. 

 1 Agarum Gmelini (after Riocreux). 2 Thallaswphyllum clathrum (after Postels and K-uprecht). Both much reduced. 



thumb, show in section a series of rings, reminding one of the annual rings of a 

 dicotyledonous stem. A long ribbon-like form, L. saccharina, is also common on 

 our shores. In other forms the frond is branched and often curiously appendaged 

 at the base. In the two genera represented in the accompanying illustration (figs. 

 375 1 and 375 ^) Agarum (from the North Atlantic) has a simple, Thallasiophyllum 

 (North Pacific) a compound frond. In both the fronds are perforated or fenestrated. 

 Lessonia (Southern Pacific) attains to tree-like dimensions and is much branched; 

 it has a stem as thick as a man's thigh. But the two most impressive genera are 

 Macrocystis and Nereocyatis. The former, which occurs throughout the southern 

 oceans and on the western coast of N. America, consists of a long stalk, sometimes 

 attaining a length of 300 metres, but in thickness not exceeding a penholder. To 



