394 EOUISETALES 



essential to a proper understanding of the embryogeny to fix the attention 

 primarily upon the origin of the apex of the axis, which is defined at a 

 very early stage in Equisetum. The epibasal half of the embryo is 

 described as dividing into octants by walls at right angles to one another : 

 one of the octants then takes the lead over the others, and it is this one 

 which gives rise to the axis, with its tetrahedral apical cell like that in the 

 mature plant : the product of this octant soon constitutes the greater part 

 of the epibasal region (Fig. 214 iv.). A little consideration of the facts 

 thus stated will show, first, that the octant-walls are the natural preliminary 

 steps to the definition of a tetrahedral initial cell centrally in the epibasal 

 hemisphere : the octant-walls might even be held to be themselves the 

 first segmentations in the definition of that cell ; secondly, that, con- 

 sistently with the initiation of a conical initial cell, the origin of the axis is 

 in the closest possible relation to the point of intersection of the octant- 

 walls, just as it is found to be in the Lycopodiales. There is, however, 

 this difference, that the apex asserts itself very early in Equisetum, which 

 is in accord with the early dominance of the axis over the appendages. 

 These arise as three (or sometimes only two) leaf-teeth, borne upon a 

 coalescent sheath, which is described as originating partly from the 

 remaining three octants of the epibasal half, but partly also from the 

 lower portion of that which gives rise to the apex of the axis itself. It 

 seems quite unnecessary in such a case as this to attempt to allocate the 

 several parts to definite octants : clearly if the leaf-sheath be partly derived 

 from the stem-octant, this is not rightly so named. Probably the allocation 

 of parts to definite octants would not have been attempted in Equisetum 

 had it not been found to apply with apparent success elsewhere, and 

 especially in the Ferns. In the present case it seems more natural to 

 regard the whole epibasal hemisphere as formative of the shoot : from this 

 the stem-tip originates at the central point by the simplest course of 

 segmentation, which happens to involve octant-walls, while the peripheral 

 region of the epibasal hemisphere gives rise to the first leaf-sheath with its 

 three teeth. The shoot thus established continues its apical growth 

 directly upwards, forming successive three-leaved sheaths, followed soon by 

 the appearance of accessory branches. The hypobasal half of the embryo 

 meanwhile becomes slightly distended, as the "foot," which remains in 

 contact with the prothallus after the young plant emerges. The root 

 originates laterally in the hypobasal hemisphere in E. arvense and palustre 

 (Fig. 214), but in E. hiemale it appears to be formed laterally at some 

 distance from the base, and even from the epibasal hemisphere. 



This embryogeny accords readily with a strobiloid theory. The apex 

 of the axis arises early at the usual point in close proximity to the 

 intersection of the first octants, and it is dominant from true first. The 

 leaves, which are minor appendages in the mature shoot, arise relatively 

 late, and are not prominent features in the embryogeny. The branching 

 is clearly accessory, as it is also relatively late in the time of its appearance. 





