THE STEM. 41 



taking a longitudinal direction. A number of raised, brown, 

 spongy-looking spots may also be observed on the surface. These 

 are lenticels or porous parts of the cork, where the cells are 

 rounded, with numerous intercellular spaces between them (fig. 

 7, 1). The bark of trees is mostly made up of bast and cork, which, 

 owing to the increase in size of the wood, are thrown off from time 

 to time in shreds or flakes, and are frequently traversed by numer- 

 ous cracks and fissures. The rugged nature of many tree-trunks 

 is, therefore, a result of secondary increase in thickness. 



PHYSIOLOGY. 



The chief uses of the stem are to display the leaves, so that they 

 may best carry on their functions of assimilation and reproduction, 

 and also to serve as a means of communication between them and 

 the roots. Like the root, the stem is a vegetative organ. An organ 

 of support requires to be more or less firm in texture, and this is 

 effected by means of the hard ligniiied tissue making up most 

 of the xylem, as well as by the sclerenchyma that may occur in 

 the bast and cortex. Collenchyma, again, helps to some extent. 

 These supporting or mechanical tissues have collectively been 

 called the stereome, and this is naturally best developed in erect 

 perennials. Weaker stems make use of the ground, other plants, 

 &c, as supports, and attach themselves to these by the means 

 described on pp. 25-26. Stems also present various protective 

 appliances. Spines, thorns, and prickles help to keep off brows- 

 ing animals, and, when closely set, repel the attacks of soft- 

 bodied creeping forms, such as snails and slugs. There may 

 also be viscid substances, excreted by glandular hairs or by the 

 general surface, which prevent wingless insects from reaching 

 the leaves and flowers. In one species of willow the stems 

 of the flowering shoots are coated by a slippery layer of wax, 

 over which no insects can pass. Again, protection is needed 

 from the weather, and this is afforded by epidermis and cork, 

 which are practically water-tight. Hairs on the stem (and leaf) 

 help to keep off wingless insects. 



Stems assist in nutrition by conveying to the leaves the water 

 with substances in solution absorbed by the root, and, on the other 

 hand, carrying the materials formed in the leaves to the plant 

 body generally. The ascending or crude sap travels chiefly (see 

 p. 18) in the cavities of the lignified wood vessels and tracheites, 

 and an active movement towards the leaves is brought about by 

 the vital activity of the cells of the medullnry rays and wood- 

 parenchyma, which act alternately as suction-pumps and force- 

 pumps. By means of the pits liquid can filter from one element 



