16 ELEMENTS OP BOTANY. 



pulley attached to its axis. Over this pulley a cord / passes. One 

 end of the cord is fastened to the tip of the stem of the plant of 

 ■which the growth is to he measured and the other end has a weight G 

 attached to it. As the plant grows the pointer L 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 L 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 w^hich 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 fpur-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 



