450 STUDIES IN SEEDS AND FRUITS 



cella, that seem fitted to withstand the conditions prevailing on 

 the surface of the moon. Even now in some of our desert 

 plants we can recognise a stage intermediate between the seed 

 and the full-grown plant, such, for instance, as in those plants 

 where, as in the West African Welwitschia, the cotyledonary 

 leaves are the only leaves produced by the plant. Cases like 

 this suggest the penultimate stage of a plant's life in a planet 

 attaining the last stage of desiccation. They would also 

 represent the first stage in plant-development, when in a 

 desiccated world inhabited only by plants in the seed-condition 

 an atmosphere began to form. As the expansion of the con- 

 ditions of existence proceeded we should obtain a state of 

 things similar to that in our own world, and plants would 

 acquire the vegetative habit familiar to us on the earth. From 

 this point of view, therefore, the development of the stem and 

 foliage would be regarded as the plant's response to the pro- 

 duction of an atmosphere. The return to the cotyledonary 

 stage would be its response to the gradual disappearance of an 

 atmosphere, until at length it would be forced back to the 

 seed-stage. 



Yet not in this way only could such changes be brought 

 about. The extension of the possibilities of growth involved 

 in the expansion of life-conditions may be in other directions. 

 That indicated by the Sequoias is in one direction, whilst that 

 illustrated by the gigantic Equisetums and Lycopods of the Coal 

 Age would be in another. We have already referred to the 

 case of the Sequoias, where existence is favoured by a bland 

 regular climate on a mountain-plateau and beneath a cloudless, 

 sky. During the coal epoch the moisture-laden atmosphere 

 and an ever-clouded sky offered favouring conditions of quite 

 another ' type. It was the expansion of the life-conditions in 

 this direction that gave rise to the huge Calamaria, Sigillariie^ 

 and Lepidodendra of the Coal Age. It is the contraction of 

 the life-conditions in the opposite direction that has resulted 

 in the production of our modern Horsetails (Equisetums) and 

 Club-mosses {Lycopods). If our Equisetums have any story to 



