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). 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 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) 
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 tracheides, 
and an active movement towards the leaves is brought about by 
means which are very far from being properly understood, though 
probably transpiration has a great deal to do with the matter 
(p. 71). By means of the pits liquid can filter from one element 
