172 EVOLUTION OF PLANTS 
times attached to it are membranaceous wings to facili- 
tate its distribution by the wind. 
Before the seed germinates, the enclosed sporophyte 
absorbs water rapidly, and the dormant protoplasm of 
its cells resumes its activity. The little plant increases 
quickly in size, growing at the expense of the food 
stored in the surrounding endosperm. The root elon- 
gates, and pushes out through the micropyle, turns 
downward, and buries itself in the earth, and thus fas- 
tens the young sporophyte into the ground. In the 
meantime the cotyledons have enlarged and turned 
green, and finally pull themselves out of the seed, whose 
empty shell is thrown aside. The young sporophyte is 
now quite independent, and in course of time assumes 
its perfect form. 
The stem of the seedling sporophyte contains a circle 
of separate vascular bundles, not unlike those in the 
stems of some Pteridophytes, but there is soon devel- 
oped in each bundle a zone of growing tissue, finally 
connected with that of the other bundles by means of 
a similar zone developed in the tissue lying between 
the separate bundles. This zone of growing tissue, or 
“ cambium,” characterizes the older stems of all Conifers, 
and to its continued activity is due the annual growth- 
rings found in these trees. A similar secondary growth 
in thickness is known to have taken place in the stems 
of the fossil Lepidodendrons, which it has been suggested 
may have been the progenitors of the modern Conifere. 
.. Unless injured, the original stem-apex of the embryo 
persists in the older sporophyte, and to this is due, as 
we have said, the extraordinary height which some of 
the Conifers attain. 
