130 



A TEXTBOOK OF THEORETICAL BOTANY 



of the thyroid gland, and it is a noticeable fact that goitre is almost unknown 

 among the Japanese. Apart from their use as food the kelps are perhaps the 



Fig. 



115 . — Laminaria digitota. 

 Entire plant. 



Fig. 1x6. — Lamitiaria hyperborea. Plant 

 showing development of new frond 

 at base of old frond. 



most important commercial source of Iodine, which is extracted from the 

 ash after the weeds have been burned. 



Structure of the Thallus 



The thallus of Laminaria shows an exceptionally complex structure, 

 the most elaborate in any group of the Algae. Anatomically stipe and 

 lamina are alike and both show a separation into three distinct zones (Fig. 

 117) although these are more clearly marked in the stipe owing to its greater 

 thickness. The stipe may also show annual zones of growth. Near the apex 

 the blade is only one cell thick, but it soon becomes two-layered, after which 

 the primary tubes are formed, which constitute the medulla and separate 

 the two external layers. By division of the cells of the outer layers parallel 

 to the surface a zone of cortex is cut oJflF. These cortical cells then elongate 

 longitudinally and the common walls between them swell and so separate 

 the cells from one another, except at certain points of union where they 

 become drawn out into short secondary tubes. In this way three separate 

 zones of tissue become differentiated. On the outside lies the external 

 layer, the cells of which are primarily concerned with assimilation. These 



