of Pilularia globulifera , L, 259 
Subsequent Growth of the young Plant. 
After the eighth day the first leaf grows with great rapidity, 
and soon reaches its full size, breaking through the overlying 
prothallium-cells about the ninth day. All the cells elongate 
very much, and in the ground-tissue are developed large inter- 
cellular spaces forming air-passages very similar to those in 
the root, but less systematically disposed. They are separated 
by single layers of cells, radially disposed, so that a cross- 
section (Plate XV, Fig. 6) presents the same wheel-shaped 
appearance that is observed in a similar section of the root. 
Small intercellular spaces are also formed later between the 
outer cells of the hypoderma. There is only one vascular 
bundle, and this is of a very simple character. It is surrounded 
by a bundle-sheath of small cells, whose walls color more 
intensely than those of the mesophyll. The rest of the bundle 
is composed of narrow cells, with more or less pointed ends, 
and no intercellular spaces. At two or three points are 
developed small spirally-marked tracheids, which are the first 
to be developed in the young plant. The first signs of 
the thickenings in their walls is evident about the ninth day, 
that is, about the time that the young plant breaks through 
the prothallium. 
The epidermis consists, as is usual in leaves of this form, of 
very much elongated cells. As in ferns, there is some chloro- 
phyll formed in the epidermal cells. The stomata (Plate XV, 
Figs. 12, 13) are few in number and of simple structure. 
The second leaf (Plate XV, Fig. 8), and the succeeding ones, 
exhibit perhaps more frequently than the first the growth by 
an apical cell. This develops three series of segments, each 
of which divides first into an inner and an outer cell, the first 
forming the young epidermis, the other the mesophyll and 
vascular bundle, which develop in the same way as in the 
first leaf. 
About the time that the first tracheal tissue is distinguish- 
able, the second root is formed at the base and on the under 
side of the first leaf. The apical cell is formed at some 
distance below the surface, and the root begins to grow at 
