196 THE EVOLUTION OF PLANTS 



The cone of a Horsetail has a quite peculiar 

 structure, unlike that of any other living Cryp- 

 togam. The rather robust axis bears numerous 

 crowded whorls of shield-like scales, on which 

 the sporangia are seated. Seen from the out- 

 side, only the shields are visible, packed so closely 

 together that they become hexagonal from mu- 

 tual pressure; there are often about twenty of 

 them in each whorl. The shield-like scale con- 

 sists of a stalk, attached to the axis of the cone 

 at a right angle, and a flat disc which forms the 

 shield. The sporangia are borne in a circle on 

 the under side of the disc, from five to ten in 

 number. Collectively they fill up all the avail- 

 able space between the stalks of the scales. 



It will be noticed that I have not called the 

 shield-like scales sporophylls. So far as Equi- 

 setum itself is concerned, there would be no ob- 

 jection to using this word, for though the fertile 

 scales differ much from the vegetative leaves in 

 shape, they agree with them in position and in 

 the way they develope. But when we come to 

 the fossil relatives of our Horsetails, we shall find 

 that there is some doubt whether the fertile scale 

 is really equivalent to an entire leaf, as a sporo- 

 phyll should be. 



The sporangia contain numerous spores, which 

 are all of one kind. A curious point is that each 

 spore has four long bands attached to it, which 



