CHAPTER XII. THE PTERIDOPHYTA. 



349 



It consists of compactly arranged whorls of modified leaves, each 

 of which is a flattish, usually hexagonal scale, elevated centrally 

 on a short stalk, and bearing around its interior margin the spo- 

 rangia. The spores produced in the latter are provided with 

 elaters, which being hygroscopic, coil and uncoil as the amount 

 of moisture present increases or diminishes, the movements aid- 

 ing the ejection of the spores from the sporan- 

 gia. See Figs. 555 and 556. 



All the existing species of Equisetums are 

 isosporous, though there is good reason to 

 believe that in some of the fossil members of 

 the class two kinds of spores were produced. 

 Most of the living forms are practically dioeci- 

 ous, some of the spores producing male and 

 others female prothallia. This, however, is prob- 

 ably not due to any inherent difference in the 

 spores but to difference of nutrition, the pro- 

 thallia which are well nourished producing 

 archegonia only or mainly, while those which 

 receive a deficient supply of nutriment bear 

 antheridia only. 



The forms of the prothallia in the Equi- 

 setums are usually more irregular than those of 

 Ferns, commonly developing lobes or processes 

 of various sizes. The antheridia are apically or 

 marginally situated, and the archegonia, though 

 first formed on the margin, on account of the 

 continued growth of the prothallium, come to 

 /, occupy the upper surface. The antherozoids 

 are of much larger size than those of the Ferns. 

 In other respects, the mode of sexual reproduc- 

 tion in these plants closely resembles that of 

 the Ferns, presently to be described more in 

 detail. Fig. 557 represents a fruiting plant of 

 Equisetum arvense, a species common in damp, 

 sandy soil, and which produces from its rhi- 

 zomes chlorophylless, simple stems that mature their spores 

 early in the spring and then die, and later develop freely 

 branching green stems that continue to grow during the season. 



Fir.. 557. — Fruit- 

 ing plant of Equi- 

 setum arvense ; a, 

 fruiting cone; b, 

 transverse section of 

 stem, showing hol- 

 low and circle of 

 fibro-vascular bun- 

 dles and large inter- 

 cellular spaces. 



