104 STRUCTURAL BOTANY 



will generally become females. This is interesting, for 

 here we see the beginning of sexual differentiation of the 

 prothallus, which has become quite a fixed thing in other 

 families of Cryptogams. It is now a well-established 

 fact that some of the relations of the Horsetails, which 

 lived in the very ancient coal period, were heterosporous.. 

 Evidently our living Equiseta come from some of the less 

 specialised members of the stock. 



3. THE EMBRYO 



Fertilisation, so far as is known, takes place in the 

 Horsetails in the same way as in the Ferns, but the 

 details have not been studied. In fact the whole subject 

 of the sexual reproduction of Equisetum is a difficult one, 

 for the prothalli are by no means easy to cultivate, and 

 only a few observers have succeeded in tracing the 

 whole history. The prothalli will often grow healthily 

 enough up to the time when the first antheridia are 

 formed, but then they generally begin to " damp off." 

 However, the development has been followed throughout 

 by a few botanists, so that we know how the embryo 

 arises from the fertilised ovum. The latter first divides 

 into two by a horizontal wall. The first division in the 

 upper half (that towards the neck of the archegonium) is 

 by a somewhat inclined wall, which separates the first 

 leaf from the unicellular rudiment of the young stem. 

 The latter at once cuts off tw r o segments, which give 

 rise to the second and third leaves. These three leaves 

 form the first whorl of the young Horsetail. Though 

 coherent at the base, they are more distinct from each 

 other than the leaves of later-formed whorls. After 

 these first divisions the apical cell of the stem has 



