EMBRYOGENESIS IN THE PSILOTALES AND EQUISETALES 91 



root arises from the epibasal segment (Jeffrey, 1899). In E. variegatum 

 Buchtien (1887) found a flattened embryo like that of E. debile. This 

 type of embryonic development is considered to be primitive. 



Kasyap (1914) and Campbell (1928) have noted that a gametophyte 

 of E. debile may have a large number of developing embryos, some of 

 which become abortive. Now, in Equisetum, as in other archegoniate 

 plants, the gametophyte tissue is probably affected by growth substances 

 diffusing from the developing embryo. It may be that those embryos, 

 in which the first and subsequent walls are formed in what may be 

 regarded as anomalous positions, have been affected by secretions 

 from earlier-formed embryos; i.e. the anomalous developments are 

 indicative of changes induced in the zygotic reaction system. With new 

 and refined techniques, the experimental testing of this hypothesis need 

 not be regarded as impossible. 



A well defined tetrahedral apical cell becomes functional at an 

 early stage in the embryogeny and, together with its segments, it 

 constitutes the highly regular, conical meristem characteristic of the 

 adult Equisetum shoot. At an early embryonic stage also, cells lateral 

 to the apical cell grow out and form the sheath of small scale-like 

 leaves. Usually there are three leaves in this first whorl, but there may 

 be two or four, affording evidence of the absence of an invariable and 

 exact correspondence between cellular segmentation and organ 

 inception. On the growth of the apex, other fused leaf-whorls arise at 

 regular intervals, and the articulate, or jointed, character of the 

 equisetoid stem begins to be apparent. The genetical and other factors 

 which determine the whorled phyllotaxis and the articulated stem 

 character in Equisetum are therefore active at an early stage in the 

 ontogenetic development. 



The embryological data for Equisetum are of particular interest, 

 because not only is it the sole surviving genus of the Articulatae, i.e. 

 the Articulate Pteridophyta or Sphenopsida, but it can be traced back, 

 almost unchanged, to Palaeozoic times, through fossil forms known as 

 Equisetites (Hirmer, 1927). In fact, Equisetum may be regarded as the 

 living correlative of the smaller and simpler members of a great 

 Palaeozoic group, the Equisetales. That being so, it is worthy of note 

 that the supposedly primitive embryonic organ, the suspensor, is 

 completely absent from Equisetum, and that at an early stage in the 

 embryogeny all the characteristic features of land plants can be 

 observed, viz. an axis with leaves and a root. 



The major features of the embryogeny in Equisetum occur with a 

 considerable degree of regularity and constancy, the differences being 

 of the kind that we might expect to find in related species. Thus, in 

 E. debile, the whole of the hypobasal region develops into an enlarged, 



