98 EMBRYOGENESIS IN PLANTS 



a second source of carbohydrates. From an early stage these will 

 be passing downwards into the embryonic shoot, which, as we have seen, 

 is also receiving carbohydrates from below. The constituent cells of the 

 shoot region divide, enlarge, and become the repositories of abundant 

 starch grains. In short, the swollen, tuberous structure, which we know 

 as the protocorm, is formed. These developments are attended by a 

 delay in the organisation of a recognisable and actively functioning 

 shoot apex; or it may be that a small apex is present but is masked by 

 the strong parenchymatous development in proximity. That a feeble 

 or attenuated apical meristem, readily escaping observation, is present 

 can scarcely be doubted. A further inference from these data is that, 

 during the early embryogeny of L. cenmum, either the supplies of 

 nitrogen-containing metabolites reaching the apex are limiting, or some 

 essential growth factor is lacking. Seasonal factors which restrict 

 apical growth but admit of the accumulation of carbohydrates may 

 also be involved. Some of these are known to be important in promoting 

 tuberous developments in lycopods and some other pteridophytes. 

 The development of storage parenchyma in close proximity to the 

 apical meristem may not only make the latter difficult to recognise: 

 it may also lead to displacements of the protophylls. 



That a modified shoot meristem is present throughout the embryo- 

 geny, giving rise to leaf primorida (protophylls) on its flanks, is a 

 reasonable assumption. How, otherwise, are we to account for the incep- 

 tion of the protophylls, for the inception of the apex itself, and for the 

 fact that, when a shoot apex at length becomes recognisable, the proto- 

 phylls to which it gives rise are identical with those previously formed ? 

 As a fact, when the apex becomes readily visible, it is found to be in 

 close proximity to the last-formed protophyll. Fig. 24. Inconspicuous, 

 and seemingly inactive, apical meristems are also characteristic of the 

 embryogeny of ferns such as Augiopteris (Campbell, 1921); and 

 almost unrecognisable though still functional apical meristems, in 

 contiguity with swollen storage parenchyma, are known in the vascu- 

 lated prothalli o^ Psilotum (Holloway, 1939), and have been produced 

 experimentally in the rhizome of Onoclea (Wardlaw, 1945). 



While the shoot apex remains in the inconspicuous and relatively 

 inactive phase, no vascular tissue is differentiated in the protocorm. 

 In the protophylls, each of which is formed from a foliar apex that soon 

 becomes attenuated and parenchymatous, a vascular strand may or 

 may not be diff'erentiated ; such strands as are formed end blindly in 

 the upper region of the protocorm. But later, when an actively growing 

 shoot apex is established, an axial or cauline vascular system is differen- 

 tiated, with which the vascular strands of the new leaves become 

 conjoined peripherally. This close relationship between the activity of 



