318 EMBRYOGENESIS IN PLANTS 



in size and the instability associated with the drift to cytoplasmic or 

 metabolic heterogeneity. As cell division tends to restore equiUbrium 

 in the system, the position of the partition wall will be such that the 

 forces present in the two daughter cells will be balanced. According to 

 the nature and distribution of these forces, the zygote may be more or 

 less equally divided, or it may be divided into a small, densely proto- 

 plasmic distal cell and a larger basal cell. The nature of this division 

 is thus ultimately determined by the specific constitution and meta- 

 bolism of the zygote. 



(v) During the further growth of the embryo, the positions of the 

 successive partition walls are in general conformity with Errera's law 

 of cell division by walls of minimal area. 



(vi) As the embryonic development proceeds, factors in the genetical 

 constitution and in the environment become effective; growth is 

 specifically allometric or differential ; and the embryo begins to assume 

 a distinctive form. An immense diversity of form is thus possible, but, 

 with some exceptions, e.g. colonial algae, axial development is a general 

 concomitant of the establishment of polarity. 



(vii) While the embryo is still small, it shows an acropetal gradient 

 of decreasing cell size. With the exception of those algae which grow 

 by means of an intercalary meristem, the distal region of the axis, which 

 may remain perennially embryonic, becomes organised histologically 

 as an apical growing point. 



(viii) Nutrients are taken up from the environment by the more 

 basal tissues of the embryo and translocated to the apex. Primary 

 growth is in the nature of an accretionary process, the older tissues 

 becoming firm and rigid and showing various characteristic concentric 

 and radiate differentiation patterns. 



It is thus possible to specify a number of developments that are 

 typical of the embryogeny of all classes of autotrophic plants. This 

 homology of organisation could be ascribed to (a) the origin of all 

 classes of plants from a common ancestor; (b) the morphogenetic 

 action of the same physical and chemical factors and developmental 

 relationships; or (c) the evolution of parallel or convergent genetical 

 systems. The differences between the embryos of related species, or 

 between those of species in the larger taxonomic units, are due primarily 

 to genetical factors. Thus, in many monocotyledons, the segmentation 

 pattern of the young embryo is practically identical with that of many 

 dicotyledons; but later, the distribution of growth in the embryos 

 becomes notably different, one forming two cotyledons with a terminal 

 shoot apex, the other a single terminal cotyledon with a lateral apex. 

 The unavoidable conclusion is that the different distributions of growth 

 in the two embryos are gene-determined, as is also the distinctive 



