328 



THE PTERIDOPHYTES 



angled shield-shaped top (Fig. 284, E~). A group of sporangia 

 hang down all around the stalk from the lower surface of the 

 shield, and each develops from a group of cells instead of from 

 a single cell, as in the common ferns (Filicales). The shields 

 separate from one another when the cone matures, and the ripe 

 spores escape through rents in the sporangia and sift out between 



the shields. The spores are 

 formed in groups of four 

 (tetrads) in the spore mother 

 cells. 



Each spore (Fig. 284, C, D) 

 bears four filaments de- 

 veloped from an outer layer 

 of the spore wall, which 

 splits into bands that sepa- 

 rate from one another, but 

 remain attached to the spore 

 at one point. The filaments 

 coil around it when moist, 

 but loosen and spread out 

 when dry. These movements 

 must assist the escape of the 

 spores from the sporangia. 

 The filaments also serve as 

 wings in the distribution of 

 the spores by the wind, and 



FIG. 285. Gametophytes of Equisetum 



-4, male prothallium: an, antheridium. 

 B, sperms. C, female prothallium : ar, 



archegonium.-^, C, after Hofmeister; they become entangled with 



B, after Schacht one another so that groups 



cling together and are carried away and germinate in clusters. 



The spores are of the same size, and therefore Equisetum is 



homosporous. 



318. The gametophytes of Equisetum. The spores only 

 retain their vitality for a few days. They produce green gameto- 

 phytes somewhat like fern prothallia, but very irregular in form 

 (Fig. 285, A, C), the larger with long lobes, at the bases of which 



