THE STEM. AI 
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). They take the place of the small openings or stomata that 
occur in the epidermis of the young stem. ‘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 numerous 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 lignified 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) 
may be of use in checking excessive evaporation, and they also 
cause the deposition of dew, by which the plant is cooled. 
Besides this, they 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 walls of the lignified wood vessels or tracheides, 
