

49 THE GROWTH AND PARTS OF PLANTS 
for we find them doing well there, while they cannot 
grow well in the open field. 
The duration of leaves. ‘The leaves of most plants 
live but a single season. Most trees and shrubs shed 
their leaves in the autumn. Iivergreen trees form a 
crop of leaves each season, but these leaves remain on 
the tree for more than a year, in some trees for several 
years, so that the trees are green during winter as well 
as summer. ; | 
The veins of leaves. If you examine carefully the 
leaves which you have gathered while learning the 
stories of their colour and form, you will see that all of 
them have veins, as we call them. These show espe- 
clally on the underside of the leaf as prominent raised 
lines where the leaf substance is thicker. There are 
large veins and small ones. The midrib of the leaf 
is the largest vein. The smaller veins branch out 
from the larger ones, or arise at the base of the leaf. 
If you look carefully at the leaves of some plants (£1 
toma, for example, Fig. 102), you will see that the 
smallest veins form a fine network. The entire system 
of veins in a leaf forms the skeleton of the leaf. Where 
the veins form a network the leaf is said to be net: 
veimed. Where the veins run-in parallel lines through 
the leaf the leaf is parallel-veined. 
You have observed the germination of the maize, and 
that it has one cotyledon. How do the veins in the 

