THE NUTRITION OF THE EOOT 79 



consisting of cork cells arranged more or less in rows. Between 

 these cork cells the air has free access to the cortical tissues. 

 To the naked eye these lenticels appear as minute warts on 

 the surface of the stem, and during periods of long-con- 

 tinued rain they assume a white mealy appearance (e.g., on 

 alders in the autumn, on potatoes in a wet summer). This 

 appearance is due to the excessive production and swelling up 

 of certain portions of the corky tissue, so that the outermost 

 cells split apart and the loose cork cells are pushed out of the 

 pore in the form of a loose powder. 



The rapid movement of air in the interior of stems is 

 ensured by the presence in some plants of large irregular 

 but continuous passages between the cells (in submerged 

 plants and plants inhabiting swamps), while in others the 

 older vessels, which form the centre of the woody cyclinder, 

 are filled with air. 



How eager all plants are to obtain sufficient air we can 

 judge from the behaviour of some which send up delicate 

 rootlets out of the soil into the moist atmosphere of a green- 

 house or forcing frame. In nature, too, this occurs in many 

 plants rooted in swamps. Avicennia, for instance, in its natural 

 surroundings of a mangrove swamp, sends up innumerable 

 lateral roots, which stand out above the surface of the water, 

 and are provided with lenticels for the purpose of taking in 

 oxygen. 



.Gardeners will all know well enough the upward-growing 

 roots of palms and Pandanus. The tips of these roots, which 

 project above the soil, but almost always remain short, have a 

 mealy appearance. Sometimes several inches of the root are 

 evenly covered with this powdery substance ; in other cases 

 the mealy covering is interrupted by several rings of the 

 original epidermis. The root-cap is shrivelled up to a small 

 brown cap. 



Those regions which have a powdery appearance are no 

 longer provided with an epidermis, but are covered in by a 

 loosely arranged tissue, the intercellular spaces of which com- 

 municate freely with the intercellular spaces of the cortex. 

 This layer of " spongy parenchyma " is formed in the youngest 

 portion of the root-apex by an increase in number and a 



