no THE MORPHOLOGY OF PTERIDOPHYTES 



the peculiar way in which the internodes of Equisetum are 

 formed from an intercalary meristem. If analogies are to be 

 sought with other pteridophytes, then it is not the internode, 

 but the node, which should be compared. The vascular 

 structure of the node can best be looked upon as a meduUated 

 protostele. The internodal spaces then appear as perfora- 

 tions, albeit of a pecuhar (intercalary) origin. 



Growth at the stem apex takes place as a result of the 

 activity of a single tetrahedral apical cell, daughter cells 

 being cut off in turn from each of its three cutting faces. 

 Despite the spiral sequence of such daughter cells, subse- 

 quent growth results in a whorled arrangement and three 

 daughter cells together give rise to all the tissues which make 

 up a node and an internode. It is interesting that, in the 

 first-formed stem of the young sporeUng, there are three 

 leaves in each whorl, but, nevertheless, it is stated that their 

 initiation is in no way determined by the position of the 

 cutting faces of the apical cell. Each leaf primordium grows 

 from a single tetrahedral apical cell, and in the angle be- 

 tween the leaf sheath and the axis, but on radii between the 

 leaves, lateral bud primordia arise, also with a single apical 

 cell. The lateral bud primordia subsequently become buried 

 by a fusion of the base of the leaf sheath with the axis, with 

 the result that, when it grows, it has to burst through the 

 leaf sheath, so giving the appearance of an endogenous 

 origin. However, not all branch primordia do grow, for in 

 species such as Equisetum hiemale, although present, they 

 are inhibited from growing beyond the primordial stage, 

 unless the main stem apex should be destroyed or damaged. 

 Each branch primordium, besides bearing leaf primordia, 

 also bears a root primordium which in aerial axes is also 

 inhibited from growing further. In underground axes, how- 

 ever, they are not inhibited in this way. It is interesting to 

 note that the roots which are apparently borne on a horizon- 

 tal rhizome are, in fact, borne by the axillary buds hidden 

 with its leaf sheaths, and not directly upon it. 



