32 EMBRYOGENESIS IN PLANTS 



usually in a definite relation to the axis and to the distal cell group. 

 Thus, in the embryos of dicotyledons, the spherical cellular mass derived 

 from the distal embryonic cell begins to form two equal lateral lobes: 

 these grow more rapidly than the distal cell group and are recognised 

 as the cotyledons. In lycopods, also, the enlarging embryonic tissue 

 mass gives rise to the first leaf or leaves, the inception of the first root 

 taking place more or less simultaneously. In leptosporangiate ferns, 

 the approximately spherical embryo divides in a very regular manner 

 till it has attained a certain size : thereafter it becomes possible to dis- 

 tinguish one of the quadrants as the shoot apex, one as the first leaf, one 

 as the first root, and the fourth as the formatively inactive but important 

 absorptive foot. These several organogenic developments are usually 

 accompanied by characteristic histogenic changes in the hitherto 

 uniform meristematic cells. It becomes possible to distinguish the 

 elongating cells of the incipient or prevascular tissue from the adjacent 

 incipient parenchyma, and both of these from the cells at the apices of 

 leaf, shoot and root, which remain in the embryonic state. These 

 initial organogenic and histogenic developments, which, as it were, set 

 the pattern for future developments, may well be regarded as being 

 among the most interesting, important and enigmatic phenomena in 

 biology. What factors are involved in the assumption of form, and in 

 the successive changes in form, during the ontogenetic development, 

 and hkewise in the differentiation of the increasingly complex tissue 

 systems? These are questions which we must attempt to answer, no 

 doubt very inadequately in the present state of knowledge, in the course 

 of this study. Even if we cannot explain any but a few of the pheno- 

 mena, it is important for the advancement of botanical science that the 

 relevant problems should be clearly before us ; for if we cannot under- 

 stand something of these phenomena of morphogenesis at their 

 inception in the embryogeny, how can we hope to have any exact 

 knowledge of them in the adult plant ? 



ORGANISATION IN THE YOUNG EMBRYO 



From an early stage the embryos in all classes of plants afford 

 evidence of orderly development or organisation. To understand what 

 constitutes this organisation, which is progressively manifested 

 throughout the ontogeny, is one of the central problems in biology. 

 Here, perhaps, we should note that the inception of the tissue pattern 

 in organisms is a composite effect. Thus, in any elongating organ, such 

 as a shoot or a root, the characteristic tissue pattern can be referred to 

 the axial development that follows the establishment of polarity, to a 

 concentric mode of differentiation in which we can distinguish epider- 

 mis, cortex and stele, and to a radiate mode of differentiation which we 



