FACTORS IN EMBRYOGENESIS 31 



the prothallus, but the embryo continues for some time to draw upon 

 the prothallus for its nutrition. At this stage there is no longer any 

 question of the nutrients being absorbed over the whole embryonic sur- 

 face: they must be taken up by way of the foot region; and in this we 

 probably see what is no more than the continuation of a system 

 estabhshed at a much earher stage. 



GRADIENT OF CELL SIZE 



Closely related to the topic of the preceding section is the fact that 

 the embryos of many different classes show a more or less well marked 

 gradient of cell size from the base or foot into the meristematic shoot 

 region, Fig. 7. Here we have another general phenomenon in embryo- 

 logy for which a physiological explanation is required. The data of 

 tissue culture investigations may in time afford a clue to these 

 characteristic developments. 



EFFECTS OF GROWTH AND ENLARGEMENT 



As the embryo grows, departs from the filamentous state and 

 becomes a three dimensional tissue system, the spatial relationships of 

 its parts undergo a continuous series of changes. In many of the higher 

 plants the embryo becomes approximately spherical, cylindrical or 

 obconical in shape. There is, accordingly, a continuous diminution in 

 the ratio of surface to bulk or volume. Moreover, we begin to dis- 

 tinguish the outer from the inner tissue layers, these being usually 

 arranged in a concentric manner. In a way, this may be regarded as a 

 kind of differentiation. We do not know how the outer layers differ 

 from the inner layers of an embryo — they may receive more oxygen, 

 or have a lower tension of carbon dioxide, for example — but on further 

 development they are seen to be histologically different. This matter 

 of the distribution in space and the relative positions occupied by 

 tissues certainly merits attention when we try to account for the incep- 

 tion of the several tissue systems. The progressive increase in size of 

 the embryo almost certainly has an effect on its differentiation and 

 organisation : it alters the proportion of surface to bulk and this may 

 be important where surfaces of interchange are involved and also in 

 mechanical relationships. In fact, many developing organisms afford 

 evidence of a size-structure relationship (Bower, 1921, 1930, 1948), and 

 this is seen both in the external configuration and the disposition of the 

 tissues. These relationships have their inception in the embryogeny. 



ORGANOGENESIS AND HISTOGENESIS 



As the embryo of a vascular plant enlarges, new and critical 

 developments begin to take place. In particular, new organs are formed, 



