26 



THE AMERICAN BOTANIST 



cently it has been suggested that there may be other sources of 

 nitrogen in the soil — the amino acids, for instance. In the case 

 of the bacteria which dwell in the root tubercles — those of the 

 Leguminosae are classic examples — the nitrogen compounds 

 turned over to the plants are known to be of this character, and 

 the fact that various grasses grow most vigorously when sown 

 with clover seems to indicate that these plants, too, may not re- 

 quire nitrogen in the form of nitrates but secure it in other 

 forms from the clover bacteria. 



Origin of Stipules. — In many plants the base of the 

 petiole supports a pair of small organs called stipules. These 

 may resemble the leaf-blade in texture, or thev may be tendrils, 

 sheaths, ligules, prickles and the like. The nature of these 

 structures has long been a puzzle to botanists. They have been 

 variously regarded as accessory leaves, as independent organs, 

 as the remains of two lateral leaflets and as the remains of the 

 leaf-sheath. A recent article in the American Journal of 

 Botany suggests that stipules are really homologous with the 

 teeth or lobes of leaves. A truly remarkable correlation has 

 been found to exist between stipules and notched leaves as well 

 as with multiple leaf-traces in the nodes of the stem. ]\Iost 

 students of botany are aware that the veins of the leaf are con- 

 tinuous with the flbrovascular bundles of the stem. At the nodes 

 the bundles send certain branches out to the leaves and these, 

 seen in sections of the node, are known as leaf-traces. ^lany 

 plants have a single leaf-trace for each leaf, a larger number 

 have three leaf-traces and still others have more than three. In 

 practically all plants Avith a single leaf-trace, stipules are absent 

 and the leaves are entire: in those with more than three leaf- 

 traces the leaves have a sheathing base, and in those with three 

 leaf-traces most of the plants have stipules. Notched leaves and 

 stipules seem to be related. For example, in the plants with 

 three leaf-traces those which have entire leaves have no stipules 

 and those whose leaves are notched or lobed almost iuA'ariably 



