2 8o PLANT LIFE. 



or archegonia are produced. But the Prothallium is 

 either male or female and forms either antheridia or 

 archegonia, but not both. The fertilised egg-cell of the 

 archegonia will of course produce the Equisetum. 



From the preceding, it is clear that both spores and 

 prothallia may be of different sexes ; and it is also 

 obvious that the difference between the sexes proceeds 

 further back as the plant becomes more highly special- 

 ised. In the Fern and Clubmoss, the same prothallium 

 bears male and female organs ; in Equisetum the pro- 

 thallium is male or female ; in Selaginella the spore is 

 male or female ; in flowering plants we find male spore- 

 leaves or stamens^ and female spore-leaves or carpels. 

 There is even a higher degree of differentiation still, for 

 in dioecious forms such as the Walnut, the plant itself is 

 male or female producing stamens only, or carpels only. 



In the higher flowering plants the prothallium is 

 almost entirely suppressed ; the growth of the pollen- 

 tube on the stigma is all that is left of the germination 

 of the male spore ; and in the ovule, the embryo sac 

 probably corresponds to the female spore. Selaginella, 

 and certain very rare Water Ferns show certain transi- 

 tional states between a free prothallium and the reduced 

 type found in flowering plants. 



It will be seen that the chief importance of Ferns 

 and Fern allies lies in their transitional character. 

 Their past history is of the very greatest interest, 

 because in the Devonian and Carboniferous periods, 

 these Ferns and their allies were the dominant 

 forms of vegetation. A Coal-measure landscape is 

 very difficult to realise, and the following description 

 must be taken as merely the writer's personal views. 



The atmosphere was saturated with moisture, and 

 the land covered with swamps of deep mud or ooze, 



