THE LEAF. 



175 



South African species produce them normally in the 

 adult plant. 



The appearance in various species of Fragaria and 

 in Potentilla thuringiaca of auricles on the petiole, 

 thus reproducing in these plants abnormally what is 

 a normal and fixed character in other members of the 

 order, is a most important point. In view of the fact 

 that in the two closely allied orders, Rosacese and 

 Leguminosse, pinnate leaves are so widely spread and 

 are more common in the tribes Potentillese, Spirseege, 

 Poterieee, and Roseee than are ternate leaves ; and 

 remembering also how great a role reductive pro- 

 cesses have played in the course of evolution, this 

 phenomenon in Fragaria, etc., may be regarded as a 

 case of reversion. But these extra leaflets are not 

 produced by fission of any of the normal leaflets, but 

 arise independently on that part of the petiole where 

 it may be supposed that the leaflets have completely 

 vanished in the past. 



As regards the appearance of the leaflet in the 

 basal gap of the elm-leaf described above, this is 

 probably a reversion ; on the vigorous vertical shoots 

 produced as a result of lopping branches of the 

 common and wych-elms, the large leaves are usually, 

 or very often, perfectly symmetrical, of a character 

 which is probably correlated with the vertical direc- 

 tion of growth of the shoot. It is well-known that 

 vigorous shoots, produced as a result of lopping, tend, 

 in many plants, to develop foliage exhibiting ancestral 

 characters. 



12. Rejuvenescence of Foliage. — In many plants 

 the foliage-leaves produced during the early period 

 of life are different from those produced during later 

 life. This we see in the contrast between the et pri- 

 mordial leaves" and the " needles" in the pine and 

 the larch, where they differ both in their form and 

 their position. After the second or third year the 

 primordial leaves cease to be formed, their place 

 being taken by scale-leaves and needles. However, 



