500 A TEXTBOOK OF BOTANY TCh. XII 



base are the rounded sporocarps. These contain two kinds 

 of sporangia with two kinds of spores. The megaspores 

 develop a small prothallus bearing one archegonium, and the 

 microspores a small prothallus having antheridia with large, 

 spiral, free-swimming sperm cells. After fertilization the 

 embryo develops by aid of food stored in the megaspore, 

 an arrangement which approaches very close to that found 

 in the seeds of the higher plants. Another common form 

 is Salvinia, which is small and floating (Fig. 353). At each 

 internode occur three leaves, of which two are floating and 

 entire, while one is submerged and cut to many fine, hairy, 

 root-like, pendent divisions, among which occur the sporo- 

 carps. These contain two kinds of sporangia and spores, 

 whereof the development is like Marsilia in principle though 

 not in detail. Another allied form is Azolla, with much 

 smaller floating leaves, in lobes of which are hollows occupied 

 regularly by filaments of Anabcena. Herein we have an 

 example of an endophyte (page 401), though its relation to 

 the economy of the Azolla is uncertain. 



Class 2. Equisetine^e : the Horsetails, or Scouring 



Rushes 



These plants are in part rush-like, and in part slender, ex- 

 current, and bushy, as expressed in their English name. They 

 grow along water margins, but also in sandy ground, being 

 among the first plants to spring up along railway embank- 

 ments. Those of temperate regions are never more than 

 a few feet high, but a tropical kind, leaning on other 

 vegetation, reaches forty feet. They number only 25 species, 

 all in one genus Equisetum, and their economic uses are minor. 

 Of fossil forms a great many are known, of which some grew 

 to great trees, especially in the Carboniferous Period; 

 and our existent herbaceous kinds are their degenerate 

 descendants. 



The sporophyte consists of creeping, perennial rootstocks 

 from which rise slender, erect, hollow, fine-fluted stems, on 



