100 BOTANY AND PHARMACOGNOSY. 



free from nitrogen salts. Because of this power the plants of this 

 family are useful in restoring worn-out land, i.e., land in which 

 the supply of nitrogen is exhausted, and they thus play an impor- 

 tant role in agricultural pursuits. 



The enriching of the soil is accomplished by ploughing under 

 the leguminous crops, as of clover or alfalfa, or allowing the 

 nodule-producing roots to decay, When the nitrogen compounds 

 are distributed in the soil. 



II. THE STEM. 



The stem, or ascending axis of the plant, usually grows in a 

 direction opposite to that of the root, seeking the light and air. 

 The tendency of the stem to grow upward is characteristic of 

 the majority of plants, and is spoken of as negative geotropism. 

 The growing point of the stem is at the apex, and it is protected 

 by a layer of bud scales (Fig. io8, B). 



Stems are further characterized by bearing leaves, or modi- 

 fications of them. The leaves occur at regular intervals in the 

 same species, and that portion of the stem from which they arise 

 is spoken of as a node, while the intervening portion is called an 

 internode. - 



Stem Branches usually arise in the axils of the leaves, 

 first appearing as little protuberances, sometimes spoken of as 

 primordia, on the stem. Their origin differs from that of the 

 root branches, in that they arise from meristematic or embryonic 

 tissue (p. i8i) developed just beneath the epidermis. The 

 branches, like the main axis, manifest negative geotropism. 

 although to a lesser degree. They likewise possess a growing 

 point at the apex, covered with embryonic leaves (Fig. io8). 

 Not infrequently more than one branch arises in the leaf axil. 



Buds may be defined as undeveloped shoots in which the 

 foliage is yet rudimentary. The buds at the ends of stems or 

 branches are known as apic.\l, or terminal buds, and those situ- 

 ated in the axils of the leaves, as axillary buds. In some cases 

 they are protected by scales, as in hickory, when they are known as 

 scaly buds ; while buds which are not thus protected, are called 

 naked buds. They are further distinguished as leaf, flower, and 

 mixed buds, as they develop into leaves, or flowers, or hoih. 



