EMBRYOGENESIS IN THE LYCOPODINEAE 105 



compared with those at the apical or distal end. Thus 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 

 size from the proximal to the distal region. Collectively, these observa- 

 tions suggest that, from a very early stage in the embryogeny, the 

 relation of the distal to the basal region of the embryonic axis is 

 essentially the same as that between the distal and proximal regions 

 in a normal adult shoot, i.e. the former remains meristematic and 

 draws upon the latter for its nutrients. A growing body of experimental 

 evidence supports the view that the apex of a normal shoot is a self- 

 determining, morphogenetic region ; the data given above indicate that 

 this relationship is probably established at a very early stage in the 

 embryogeny. 



On further growth the apex of the endoscopic embryo bursts out of 

 the prothallus, but the embryo may continue for some considerable 

 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 surface : 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 established at a much earlier stage. 



In considering the growth of embryos in species of Lycopodium two 

 conditions may be envisaged: (i) the embryo, under favourable 

 environmental conditions, receives a normal, balanced nutrition; i.e. 

 the supply of nutrients, including growth-regulating substances, and the 



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— ratio, are such as to permit of continuous meristematic activity at the 



shoot apex, and the further growth and differentiation of tissues and 

 organis below the apex, so that an elongating leafy shoot is produced; 

 (ii) the embryo does not receive a balanced nutrition and/or environ- 

 mental conditions may be unfavourable. In the second case it is 



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conceivable that a high — ratio, an inadequate supply of some essential 



growth-regulating substance, or some unfavourable environmental 

 condition, might modify the character of the growth in different regions 

 of the embryo. The early distension of meristematic tissue in the foot 

 and shoot regions of L. clavatum and L. cermmm respectively is 

 indicative of abundant supplies of carbohydrate to the young embryo ; 

 and this, as we have seen, goes hand-in-hand with a delay in the 

 organisation of a 'normal' apical meristem and the formation of an 

 elongating leafy shoot. 



These characteristic parenchymatous developments are exemplified 

 by such subterranean species as L. clavatum, L. volubile, L. scariosum. 



