EMBRYOGENESIS IN THE BRYOPHYTA 81 



as the characteristic polarity, the different developments of the epibasal 

 and hypobasal regions, the development of a characteristic histological 

 pattern including the estabhshment of a growing apex, and the brevity 

 of the vegetative phase in the sporophytic development. 



In the bryophytes the archegonium is typically a stalked organ, 

 otherwise free of tissue on all sides. The zygote typically divides by a 

 transverse wall, except in the Anthocerotae where it usually is a longi- 

 tudinal one, and the embryogeny is invariably exoscopic. In Anthoceros, 

 with its embedded archegonium, the constitution and metabolism of 

 the ovum and zygote are evidently considerably different from those of 

 other bryophytes. Also, contrary to Bower's generalisation, the first 

 partition wall is not at right-angles to the polar axis. In the absence of 

 experimental data, the polarity and exoscopic embryogeny of the 

 bryophytes (other than the Anthocerotae) may be provisionally 

 attributed to metabolic gradients, which have their source in the 

 gametophyte, acting on the ovum or zygote. If we regard the unfertihsed 

 egg as an initially homogeneous reaction system (see Chapter III), 

 then the stimulus of fertilisation and the impact of acropetal gradients 

 might well explain the transition to a heterogeneous system, the meta- 

 bolic complex at the apex of the zygote being different from that at its 

 base. The hypobasal region of the embryo, which lies closest to the 

 source of nutrients, typically undergoes limited growth and a parenchy- 

 matous development, whereas the distal region remains embryonic and 

 capable of further growth and formative activity. In these organo- 

 graphic developments and relationships, the bryophytes resemble 

 pteridophytes such as Psilotum and Equisetum {see Chapter VI) and, 

 indeed, vascular plants in general. 



The characteristic segmentation pattern in the several orders is the 

 result of a sequence of regular cell divisions. These take place in general 

 conformity with Errera's law. There are, of course, cell divisions which 

 do not appear to conform with this principle. But here the reader 

 should be reminded that, as the embryonic development proceeds, the 

 distribution of metabolites becomes progressively more heterogeneous, 

 new factors become incident and new forces come into play, and these 

 are all liable to have an effect on the nature of cell growth and division. 

 As each genus has a characteristic protoplasmic constitution and 

 metabohsm, differences in the cellular pattern are to be expected; 

 but, as we are concerned with a group of related organisms, these 

 differences will not be of a major kind. These views are in keeping with 

 those of the older anatomists who held that the mode of segmentation 

 in the embryogeny had taxonomic significance, but here the part 

 played by physical factors is given greater prominence. 



Contemplation of the embryonic and sporogenous development of 



