106 STRUCTURAL BOTANY 



of the archegonium. The young plant is now becoming 

 independent, though for a time it remains connected, by 

 means of the foot, with the prothallus. The main axis 

 formed directly from the embryo has only a limited 

 growth. It remains very slender, and stops growing 

 after it has formed from ten to fifteen whorls of three 

 leaves each. It is interesting to note that in the 

 smallest species of JSquisetum, E. scirpoides, three-leaved 

 whorls are formed throughout life. At the base of the 

 main stem a stouter lateral branch arises, and this again 

 produces another still more vigorous shoot, and so on. 

 Thus the mature form of the species is gradually built up 

 by the production of successive branches, each more 

 highly developed than the last. After a time one of 

 the lateral shoots turns downwards and penetrates the 

 ground, thus forming the first rhizome of the young 

 plant. The main root is fairly well developed, though 

 only a temporary organ, and shows the same structure 

 as the subsequent adventitious roots. 



SUMMARY 



We have now traced a Horsetail through the com- 

 plete cycle of its life. So far as the general course of 

 development is concerned, we have found an essential 

 agreement with that in the Ferns namely, a sharply 

 marked alternation of generations, spores of one kind, 

 and a well- developed prothallus. Although the latter is 

 usually dioecious, the distinction between male and 

 female prothallus is not a fixed one, but is dependent 

 to a great extent on external circumstances, especially 

 nutrition. The prothallus is not unlike that of some 

 Ferns, but the plant the asexual generation is as 



