Life-History of Isoetes . 249 
cells, and encroaches upon the lower cells of the pro thallium, 
which are destroyed by its growth and their contents absorbed 
to supply nourishment to the rapidly growing embryo ; and 
by the time that the young plant breaks through the prothal- 
lium, and the root reaches the ground, the foot has grown so 
as to nearly fill the cavity of the spore, and nothing is left of 
the prothallium except a layer of cells surrounding the central 
part of the young plant. These cells have now lost their 
dense contents, and contain little else than a watery cell- 
sap. 
Subsequent Growth of the Young Plant. 
About the time that the young plant breaks through the 
prothallium, the second leaf begins to develop. The growing- 
point of the stem (Fig. 45, st) lies in the groove between the 
base of the root and leaf, and is a nearly flat area, whose 
surface is nearly at right-angles to the axis of the leaf. The 
second leaf (L 2 ) arises as a slight elevation on the side oppo- 
site the first leaf. From the first it consists of several cells, 
and its growth is entirely similar to that of the first leaf, which 
it resembles in all respects. The ligule begins to develop 
while the leaf is still very small. Almost as soon as the leaf 
can be made out, a line of procambium cells is formed, run- 
ning from the junction of the first leaf and root and continued 
into the second leaf as its plerome. As in the first leaf, the 
apical growth is of short duration, and the subsequent increase 
in length is mainly due to the rapid multiplication of the cells 
near the base of the leaf. 
The growth is slow for a time, but after reaching a certain 
length it elongates very rapidly, and about this time the first 
trace of the second root appears. It arises at the base of 
second leaf in the immediate vicinity of the common fibro- 
vascular bundle of the stem. A group of cells (Fig. 47, r 2 ) here 
begins to multiply actively, and very soon shows a division 
into the initials of the tissue-systems of the young root. As 
the growing cells of the young root stain more deeply than 
those of the surrounding tissues, it is distinguishable at a 
T 2 
