236 THE EVOLUTION OF PLANTS 



The Seed in its earliest known forms was 

 an elaborate structure, much more so, in fact, 

 than in most recent plants. The pollen-chamber 

 and the very complete system of vascular strands 

 are characteristic features. That the mechanism 

 for the reception of the pollen-grains worked 

 well is shown by the great number of grains 

 (100 or more) sometimes found within the pollen- 

 chamber. Whether so many could have been 

 simply brought by the wind or whether insects 

 were already engaged as pollen-carriers is an open 

 question. The complexity of Palaeozoic seeds 

 was almost certainly connected with the sperma- 

 tozoid method of fertilisation, which doubtless 

 then prevailed, and which still lingers in the 

 Cycads and the Maidenhair-tree. Room had to 

 be provided for the movements of the actively 

 swimming sperms discharged from the pollen- 

 grains. Water for them to swim in was also a 

 necessity, and most likely the vascular system of 

 the seed served to supply it. In later days the 

 evolution of the pollen-tube allowed of a consider- 

 able simplification of the seed, or rather of the 

 ovule. But under the circumstances which pre- 

 vailed in early periods the Palaeozoic seed was 

 evidently a very perfect piece of mechanism. 



The subject of adaptation in fossil plants is 

 much too wide to be pursued further here. Their 

 study has made an interesting contribution to the 



