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 sporpphyte 

 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 sporophj^te 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 steins 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 Coniferae. 



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. 



