16 ELEMENTS OF BOTANY. 
pulley attached to its axis. Over this pulley a cord f passes. One > 
end of the cord is fastened to the tip of the stem of the plant of 
which the growth is to be measured and the other end has a weight G 
attached to it. As the plant grows the pointer Z descends on the scale. 
The actual rate of growth is obtained by multiplying the distance which 
the pointer travels over the arc by the fraction which expresses the ratio 
of the half-diameter of the pulley to the length of Z from arc to pivot. 
Contrast the mode of growth of the root and the stem and 
try to give a reason for that of the root. 
20. The First Leaves.—The cotyledons are, as already 
explained, the first leaves which the seedling possesses, — 
even if a plumule is found well developed in the seed, it was 
formed after the cotyledons. In those plants which have so 
much nourishment stored in the cotyledons as to render these 
unfit ever to become useful leaves, there is little or nothing in 
the color, shape, or general appearance of the cotyledon to 
make one think it really a leaf, and it is only by studying 
many cases that the botanist is entitled to class all cotyle- 
dons as leaves in their nature, even if they are quite unable 
to do the work of leaves. The study of the various forms 
which the parts or organs of a plant may assume is called 
morphology; it traces the relationship of parts which are 
really akin to each other, though dissimilar in appearance 
and often in function. In seeds which have endosperm, or 
nourishment stored outside of the embryo, the cotyledons’ 
usually become green and leaf-like, as they do, for example, 
in the four-o’-clock and the morning-glory, but in the seeds of 
the grains (which contain endosperm) a large portion of the 
single cotyledon remains throughout as a thickish mass buried 
in the seed. In a few cases, as in the pea, there are scales 
instead of true leaves formed on the first nodes above the 
cotyledons, and it is only at about the third node above that 
leaves of the ordinary kind appear. In the bean and some 
- other plants which in general bear one leaf at a node along 
the stem, there is a pair produced at the first node above 
