THE EQUISETACE^E. 15 



male and others only female prothallia. It is impossi- 

 ble, before germination, to discover which is which, and, 

 since the male prothallium is always the smaller, it is 

 conjectured that the difference in the sex may be due 

 entirely to the amount of nourishment the prothallia are 

 able to obtain, those well nourished becoming female 

 and the others male. It is interesting to know that the 

 cones of fossil species have been found so well preserved 

 as to show that the ancestors of our present plants had 

 spores of two kinds, like the modern selaginellas and 

 isosetes. 



The scouring-rushes range in height from a few inches 

 to thirty feet, but the largest seldom exceed two inches 

 in diameter, and the majority are rarely thicker than a 

 lead pencil. In all, the rootstock at least is perennial, 

 and in some the aerial stems last for several years. 

 There are about twenty living species, yiost of them in 

 temperate regions. During the Carboniferous Age they 

 were most abundant, almost equalling the ferns in num- 

 bers. Though few in species now, they are abundant 

 and widely distributed. North America* contains repre- 

 sentatives of more than half of all the species in the 

 world. 



The name Equisctum is from two Latin words mean- 

 ing " horse " and ''bristle," and was applied to these 

 plants in allusion to the many slender branches of cer- 

 tain species, which render the plants not unlike the tail 

 of ahorse in appearance. In the order Equisetales there 

 is but one living family, the Equisetaceae, and in this 

 family there is but the single genus Eqnisetum. The 

 fossil species mostly belonged to the family Calamari^e. 

 Many of them attained the size of trees. 



