THE VASCULAR CRYPTOGAMS 



103 



It is remarkable that while the Horsetail plant bears 

 no resemblance whatever to a Fern plant, the prothallus 

 and sexual organs are so much alike in both. It is true 

 that they differ a good deal from those of the Male Fern, 

 but other Ferns come very near the Horsetails as regards 

 their sexual generation. 



Whatever view we take of the origin of the two 

 generations in the Vascular Cryptogams, there is no 

 doubt that the sexual a.n. 



generation is much the 

 more primitive, i.e. the less 

 altered, of the two. Con- 

 sequently we find that at 

 this stage there is much 

 in common between families 

 which, so far as their sporo- 

 phytes are concerned, have 

 lost all traces of relation- 

 ship. 



The most striking point 

 about the prothallus of the 

 Horsetails is its being 

 usually (though not without 

 exception) dioecious. We found that in the Ferns very 

 small prothalli often form male organs only, while the 

 better-grown individuals produce archegonia as well. 

 In Horsetails this difference has gone further and 

 become more constant. Even in Horsetails, however, it 

 is not fixed, but depends a great deal upon nutrition. 

 Prothalli grown on a bad soil (e.g. damp sand) will only 

 produce male organs, while those which are better treated 

 and provided with plenty of food (say in the shape of a 

 food-solution such as that described in Part I. p. 200) 



FIG. 47. Equisetum maximum ; 

 fertilised archegonium. a.n, neck 

 ofarchegonium ; em, young embryo, 

 showing first divisions. Magni- 

 fied about 150 diameters. (After 

 Buchtien.) 



