STUDIES AMONG OUR COMMON HEPATIC^. pp 



thicker. In color the plants are a darker or lighter green according 

 to the degree of shade in which they grow; when exposed to direet 

 sunlight, they become tinged with yellowish-brown. The stem itself 

 is in the form of a cylinder, which is slightly flattened from above 

 downward; from the lower surface is given off a thick felt of pale 

 rootlets which soon lose their living contents and apparently serve 

 simply to hold the plant in place. If the stem is studied in thin 

 transverse and longitudinal razor-sections, which can readily be cut 

 in pith, it will be found to be built up of a mass of cells which are 

 closely packed together without any intercellular spaces, a well- 

 developed stem being fifteen or more cells thick. The cells making 

 up the interior of the stem are considerably larger than the others 

 and are three or four times as long as they are broad; they have 

 colorless contents and thin, transparent walls, and they fit together 

 by blunt ends. On the upper surface, there is a single layer of 

 short green cells whose outer walls are thickened, but not pigmented, 

 while, on the lower surface, there is another layer of short cells with 

 thickened walls, which usually become reddish as they grow older. 

 The rootlets proceed from the cells of this layer as simple, hollow pro- 

 jections, and the interior cells neighboring them frequently exhibit, 

 to a less extent, a similar thickening and pigmentation of their walls. 

 The stems normally branch by forking. The leaves are alternate and 

 are arranged in two rows, one on each side of the stem. They are 

 attached by a very broad base, and the line of insertion is strongly 

 oblique and curves about the stem in such a way that the lower 

 margin of the leaf is toward the apex of the stem. When the leaves 

 are close together, as is frequent in this species, the lower part of 

 each leaf comes to lie underneath the upper part of the succeeding 

 leaf, an arrangement which is called " succubous " and which is of con- 

 siderable systematic importance. At the stem-apex where the leaves 

 are undeveloped and closely crowded, the stem curves slightly upward 

 and lifts the apical bud away from the substratum. The leaves are 

 broad, more or less quadrate in outline, and, at the broad apex, divided 

 into three or four obtuse or subacute teeth or lobes, which extend 

 about a quarter of the distance toward the stem and are separated 

 from one another by broad sinuses. Except for these lobes the mar- 

 gins are entire and are free from appendages of any sort. The lower 

 margin is often reflexed near the base. The leaf -cells are arranged in 

 a single layer, except very close to the line of insertion, where they 

 are in two. They are polygonal in outline and do not vary greatly in 

 size in different parts of the leaf. The cell-cavities are separated from 

 one another by thin walls, which become slightly thicker where three 

 cells meet, forming inconspicuous " trigones." The marginal cells are 



