FACTORS IN EMBRYOGENESIS 29 



POSITION AND NUTRITION 



As the embryo grows and develops, its nutrition becomes of ever- 

 increasing importance. Tlie nutrition of an enclosed embryo must 

 stand in some relationship to the position which it occupies in the 

 gametophyte tissue. This question will be considered in greater detail 

 in subsequent chapters. As already noted, the gradients of nutrient 

 supplies to the young embryo may be among the factors determining 

 its polarity. From a very early stage, the distal pole becomes the 

 primary growing and formative region, whereas the proximal or basal 

 pole becomes parenchymatous and functions as the region of uptake 

 of nutrients from the gametophyte. In archegoniate and seed plants, 

 there is also evidence that from an early stage nutrients may not be 

 absorbed over the entire surface of the embryo, as might perhaps be 

 supposed, but are taken up by the basal region and passed on from there 

 to the distal growing region, just as in the adult shoot. Even in fila- 

 mentous algae there are indications of the same general mechanism of 

 growth. 



In bryophytes and in Psilotum, Tmesipteris and Equisetum, where 

 the embryogeny is exoscopic, the hypobasal region remains in contact 

 with the gametophyte tissue and acts as the absorptive foot region, 

 taking up nutrients and passing them on to the actively meristematic 

 epibasal region. We should not, indeed, expect the epibasal cell, which 

 lies in contiguity with the archegonial neck and may soon emerge from 

 the gametophyte tissue, to be the absorptive region of the embryo. 

 All this seems to be straightforward and in conformity with our 

 general knowledge of axial growth. Nevertheless, it is doubtful if such 

 reasoning, based on a consideration of the anatomical data, goes to the 

 root of the matter; for in endoscopic species, e.g. Lycopodium, the 

 suspensor and the nutrient-absorbing foot are in contiguity with the 

 archegonium neck and the growing embryo becomes deeply embedded 

 in the fleshy tissue of the prothallus. Hence we may ask: Do these 

 embedded endoscopic embryos absorb nutrients over their entire 

 surface, or is the uptake confined to the foot? While the embryo is 

 still very small, it may be that nutrients are absorbed over the whole 

 of its surface; but, as it enlarges, the indications are that the foot 

 is the main absorptive region. Certainly, from an early stage, there 

 is a marked difference in the development of the basal cells as compared 

 with those at the apical or distal end. The cells of the foot soon become 

 considerably distended; they may extend what appear to be suctorial 

 processes into the tissue of the prothallus ; they may contain deposits 

 of starch grains ; and they show a gradient of cell-size from the proximal 

 to the distal region. Collectively, these observations suggest that, from 



