EMBRYOGENESIS IN THE LYCOPODINEAE 101 



arises alongside the third and forms a pair with it, also develops a 

 swollen base. Holloway noted that in no case 'was the prothallus 

 found attached to a young plant which has more than two original 

 protophylls.' From this observation it can be inferred (i) that the 

 enlargement of the protocorm is the result of photosynthetic activity in 

 the protophylls, and (ii) that, from an early stage, the sporophyte must 

 depend for mineral and nitrogenous nutrients on its basal rhizoids. 

 The 'protocormous rhizome' continues to elongate laterally in relation 

 to the further development of protophylls which may be 8-12 in number. 

 Fig. 23. The protophylls arise in pairs along the dorsal side of the 

 rhizome, i.e. there is a certain regularity in the positions in which they 

 are formed. This suggests that an apical meristem is present, even 

 though it may be feeble and inconspicuous. Each protophyll has a 

 vascular strand which ends blindly in the dorsal parenchyma of the 

 protocorm. This organ, in fact, consists of parenchyma throughout. 

 Eventually the shoot apex becomes an actively growing and readily 

 distinguishable region. Its position and activity are indicated by (i) 

 the aggregation of several protophylls, (ii) the differentiation of a 

 cauline vascular strand immediately below it, (iii) the development of 

 the first root on the distal (or growing) end of the protocorm, i.e. the 

 side remote from the foot, and (iv) by the differentiation of vascular 

 tissue between shoot and root. Soon after this stage has been reached, 

 the embryo grows into a recognisable plantling of L. laterale, the 

 elongating shoot axis bearing leaves which are 'in no wise different 

 from the protophylls.' This statement gives further support to the 

 view that the protophylls of the protocorm are formed in relation to a 

 shoot apex. 



The comparative inactivity of the shoot apex in the early embryo- 

 geny of L. laterale, as in L. cernuum, is associated with a phase of 

 metabolism in which carbohydrate synthesis is predominant. The 

 protophylls, however, yield evidence of some apical growth; they 

 elongate into needle-like structures and each has a vascular strand. 

 Why the apex of a leaf primordium should show active growth while 

 the shoot apex remains relatively inactive is a phenomenon for which 

 no adequate explanation has so far been advanced. 



A comparison of L. cernuum with L. laterale shows that although 

 the former typically develops a small group of protophylls on the top 

 of the protocorm, some lateral development may also be observed. 

 L. ramulosum resembles L. laterale, but, in addition, may develop 

 swollen bulbils on the protocorm (Holloway, 1915). 



In some other species of Lycopodium the foot may be partly pene- 

 trated by vascular tissue. L. complanatum (Wigglesworth, 1907) and 

 L. volubile (Holloway, 1915) may show such developments. This may 



