100 EMBRYOGENESIS IN PLANTS 



the apex and the inception of vascular tissue is general in pteridophytes 

 (Wardlaw, 1944). 



The conclusion of early investigators that the protocorm of L. 

 cernuum is a shoot that has been considerably modified during its 

 embryonic phase is thus confirmed. Since the formation of a protocorm 

 is a regular and characteristic feature of L. cernuum and other species, 

 and since this development cannot be attributed solely to seasonal 

 conditions, genetical factors must be involved. As to the precise 

 nature of this genie action we have no information, but from other 

 sources we know that protein synthesis is gene-controlled and that it 

 may be delayed, impeded, or 'blocked' under environmental conditions 

 that impede or delay the action of particular genes. If we apply these 

 conceptions to what we know of the embryogeny of L. cernuum, it 

 could be inferred that the genes which determine protein synthesis at 

 the shoot apex are initially ineffective. But after the embryo has been 

 growing for some time, and possibly in relation to the photosynthetic 

 activity of its several protophylls, certain metabolites essential for 

 apical growth, but hitherto limiting, become available and normal 

 shoot growth ensues. In this interpretation, the protocorm is a 

 morphological expression of factors in the hereditary constitution, but 

 whether or not it should be regarded as a phyletically 'primitive' organ 

 is debatable. It could, in fact, be argued that the protocorm is a 

 'derivative,' though a retrograde (but non-lethal), result of genie 

 mutation or chromosome change. Whether or not the tuberous 

 development is biologically advantageous, as in the older view, might 

 well be the subject of further investigation. The view that these 

 parenchymatous swellings in lycopod embryos are biological adapta- 

 tions should, in the writer's view, be received with caution, though it 

 may well be that further investigation will show that it is not without 

 justification. In this connection, mention should be made of the fact 

 that Wetmore and Morel (1951) have successfully grown the prothalli 

 and young sporophytes of L. cernuum aseptically by the methods of 

 tissue culture. This discovery should make possible a whole series of 

 new and interesting experimental investigations. 



Lycopodium laterale. The prothallus and embryogeny of L. laterale 

 are of the L. cernuum type with the protocormous development greatly 

 increased. Fig. 23. In the early stages, a starchy tuberous protocorm, 

 surmounted by one or two protophylls, is formed, a conspicuous foot 

 being also present (Holloway, 1909). According to Holloway (1915), 

 the protocorm and protophylls (in which stomata occur) are of a vivid 

 green colour. The third protophyll arises in a lateral position and, 

 when fully grown, has a swollen base which is distinct from the original 

 protocorm though it is conjoined with it. The fourth protophyll, which 



