236 Practical Plant Biology. 



to divide, while the cytoplasm of the embryo-sac increases in bulk 

 at the expense of the nucellus and of material transported to it by 

 the conducting tract in the stalk. Cell-walls are produced which 

 subdivide the embryo-sac into nucleated cells. Thus the sac 

 becomes filled with tissue, the endosperm. Meanwhile the nucellus 

 is reduced to a membranous sheath while the outer integument is 

 hardened and becomes the seed-coat. 



Fertilisation also induces changes in the carpel which hardens 

 and becomes a membranous coating the fruit adhering to the 

 outside of the seed. 



After ripening the seed passes into a dormant state, in which 

 the suspension of vital activities seems to be complete. Thus in 

 many dormant seeds no respiration can be detected. The pro- 

 toplasm also is in a particularly stable state. In this connection 

 it may be mentioned that some seeds, when thoroughly dry, may 

 be- exposed to a temperature of over 120 C. without injuring their 

 power of germination. The addition of moisture to seeds in this 

 state of suspended animation restores to the embryo its normal 

 sensitiveness and enables it by respiration to set free energy stored 

 in its own cells and in the surrounding endosperm. The water 

 seems to act as a catalyst to the various reactions which constitute 

 embryonic metabolism. By the absorption of the endosperm 

 material is found for growth, and the root elongates and forces its 

 way through the seed-coat. In response to the stimulus of gravity 

 the emerging root .turns downward into the soil. The seed-leaves 

 are then drawn out of the empty seed and being provided with 

 chloroplasts make the young plant independent. 



Consideration of the structure and life-history of the Buttercup 

 shows that it is more nearly comparable with the Pine than with 

 the Archegoniates. The absence of cambial or secondary thicken- 

 ing makes the structure of stems and roots appear more simple, 

 but the actual composition of the conducting tissues, wood and 

 bast is more complex a greater variety of elements contribute to 

 form them. It is, however, in the reproductive organs that we 

 find the most marked differences. The events of the life-history 

 show that the pollen-grains and embryo-sacs of the Buttercup, like 

 those of the Pine, are homologous to the spores of the Arche- 

 goniates. 



Thus the stamens and carpels are respectively microsporophylls 

 and megasporophylls, and the pollen-sacs and ovules may be 

 homologised with microsporangia and megasporangia. The mi- 

 crosporophylls (stamens) of the Buttercup are, however, more 

 divergent from the sporophylls of the Ferns than are those of the 



