112 EMBRYOGENESIS IN PLANTS 



In S. denticulata (Bruchmann, 1912) the embryonic cell divides first 

 by a vertical wall into two approximately equal cells. One of these cells 

 now divides by an oblique wall, the single and persistent apical cell of 

 the embryonic shoot being thus formed, Fig. 25h-l. This apical cell, 

 in shape an inverted tetrahedron, undergoes segmentations which may 

 be somewhat less regular than those observed in similar apical cells. 

 The other segment of the embryonic cell gives rise to the first leaf; 

 the second leaf is formed from the first segment of the shoot apical 

 cell. Meanwhile, the suspensor shows some irregularity in its develop- 

 ment. On the side opposite to the shoot apex it divides into several 

 actively enlarging cells, with the result that the embryonic axis is tilted 

 at right-angles to the suspensor. Fig. 25. Although the young plantling 

 of Selaginella is sometimes described as being 'dicotyledonous', it will 

 be realised that this is not quite exact, i.e. in the sense that the term is 

 applied to flowering plants. These first leaves do not enlarge as a result 

 of apical growth but rather from the activity of marginal initial cells. 

 In S. denticulata, the large foot region is formed from the cells of the 

 suspensor adjacent to the hypocotyl. In contiguity with the latter, and 

 from the upper part of the foot, an outgrowth develops: this is the 

 primary rhizophore from which the first roots are later formed. While 

 these several organogenic activities are taking place, a central strand 

 of elongated cells, having its inception below the shoot apex and 

 traversing the hypocotyl to the primary rhizophore, is differentiated; 

 this is the incipient vascular system, or stele, of the developing axis. 

 In each of the leaves a small vascular strand is also diff'erentiated and 

 becomes conjoined with the axial stele. The embryonic development 

 in S. rubricauUs is essentially similar. 



The formation and differentiation of the embryonic parts in any 

 particular species apparently takes place with a considerable degree of 

 regularity. In S. denticulata, for example, the suspensor, foot and 

 rhizophore originate from the basal segment of the two-celled embryo. 

 But, in other species, there may be different relationships between the 

 initial segmentation and the subsequent formation of organs. These 

 diff'erences between species are of the kind that might be expected if there 

 is a specific genie control of the distribution of growth. They aff'ord 

 parallelisms with the precise embryonic segmentation patterns found 

 in many flowering plants. Thus, in S. martensii, the basal cell of the 

 two-celled embryo gives rise to the suspensor only, the large foot and 

 all the other organs being formed from the distal embryonic cell. 

 Fig. 27a; but in S. galeottei (see below), the shoot apex and first two 

 leaves can be referred to the embryonic cell, while the hypocotyl, foot, 

 rhizophore and suspensor all originate from the basal cell, Fig. 28e-k 

 (^^^ Bruchmann, 1909, 1912, 1913; Goebel, 1918; Campbefl, 1940). 



