22 HOW TO KNOW THE FERNS 



as a rule they develop on distinctive sex lines. 

 It is obviously important that the male and the 

 female prothallus should be together. The long 

 arm-like processes on the spores often link the 

 little bodies side by side, and this would be an 

 advantage. 



The spores of the Horsetails are not long in 

 developing after they have settled in a damp 

 situation. The actual forms which the pro- 

 thalli take are often very irregular. That of 

 the male prothallus is usually rather small ; on 

 the other hand, the female prothallus is some- 

 times large, and may have complicated branch- 

 ings. As in the case of the other Vascular 

 Cryptogams which have been considered, sper- 

 matozoids are produced in the antheridia. 

 These are very active, and travel through the 

 agency of water to the archegonia on the 

 female prothallus. The spermatozoids unite 

 with the various egg cells, and in this way an 

 embryo is formed which finally develops into 

 the mature plant. Owing to the fact that the 

 prothalli of the Horsetails have proved to be 

 excessively difficult to cultivate; the life history 

 has not been so completely worked out as in 

 the case of the Ferns and Club Mosses. One 

 interesting point in connection with the cultiva- 

 tion of Horsetail spores has been brought to 

 light. Whenever the spores are growing on 

 poor soil, by far the larger number of them pro- 

 duce antheridia. On the other hand, where 

 there is plenty of nourishment the tendency is 

 all the other way. The matter is of interest, as 

 it appears to show that the amount of available 

 nutriment is a definite factor in the determina- 

 tion of sex. 



