152 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



of the former having a much divided surface, whilst in the latter they are elongated 

 and undivided. The primary leaves in the two plants are nevertheless very similar. 

 We observe that the primary leaves are traversed by forking nerves, and that the leaf- 

 surface itself shows a tendency in many forms to dichotomy, but even where this is 

 not the case the manner of growth of the leaves is alike. In the leaf //, belonging 

 to Scolopendrium officinarum, a mid-rib, constructed sympodially out of the forking 

 nerves, is visible, and as the leaf develops this always becomes more conspicuous, /; 

 whilst this forking can be traced in the succeeding leaves, in many species where these 

 take on gradually a pinnate form, the pinnules, at least at the apex of the most 

 strongly developed leaves, appear more and more as lateral formations. This applies 

 also to fern-leaves in which at an early period growth by an apical cell is replaced 



by growth by marginal cells 1 . In many 

 ferns, Ceratopteris thalictroides for example, 

 the apical cell is still present even when 

 some of the pinnules are laid down, and 

 the pinnules then doubtless represent 

 lateral off-shoots of the primordium of the 

 leaf. In such cases the primary leaves must 

 evidently be considered as arrested forma- 

 tions in which the primitive growth by an 

 apical cell has passed into growth by 

 marginal cells at a much earlier period, 

 before any branching has taken place 2 . 

 It would serve no useful purpose to depict 

 here the gradual passage to the adult leaves; 

 two facts however must be mentioned : 



1. The course of the development of 

 the primary leaves, notwithstanding all ex- 

 ternal differences, conforms with that of the 

 adult leaves they are arrested formations. 



2. In support of this we have in ad- 

 dition to morphological considerations the 

 facts, that the construction of the primary 

 leaves varies, and that the higher form 



of leaf is reached the more quickly, the stronger the germ-plant is ; further, we 

 have the result of experiments, for we can interdict the leaf-formation of an adult 

 plant and bring about again the stage of the primary leaf if we place the plant 

 under unfavourable conditions. Fig. 93 illustrates this. The germ-plant repre- 

 sented here has with leaf 5 reached the type of a feathered-leaf although only 

 two pinnules are visible, but leaf 7 has taken on again the configuration of the 

 primary leaves, of leaf 2 for example. In another experiment a plant which had 

 reached the stage of a leaf with four pairs of pinnules produced thereafter one 



FIG. 93. Doodia caudata. Germ-plant. The 

 leaves are numbered in the succession of their age. 

 In leaves 5 and 6 the feathered configuration has 

 been attained, but in leaf 7 there is a reversion to 

 the configuration of leaf 2 in consequence of un- 

 favourable environment. 



1 See Part II of this book. 



2 The primary leaves of Polypodium vulgare show no dichotomous venation ; the apical cell is 

 evidently retained for a long time. 



