42 MORPHOLOGY OF SPECIES 



and only on the branches of young plants as a rule, but they are 

 developed in the same position, at the base of the branch. We 

 have, in fact, the same kind of modification in both plants, the 

 stems being ridged, and the leaf surface reduced by a gradual 

 modification of a trifoliate to a simple form, and this has gone 

 some steps further in Ulex than in Cytisus by the further reduc- 

 tion of the simple leaves to spines. This all points to a common 

 ancestry for the two plants, and their development from a form in 

 which all the leaves were trifoliate, as occurs at the present day 

 in closely allied species, such as the common Laburnum of 

 our gardens, in which all the leaves are trifoliate from the 

 beginning. 



The formation of a number of trifoliate leaves on the seed- 

 lings and young branches of Ulex is probably, then, a survival of 

 the ancestral structure, and when a large number are produced we 

 may regard it as a reversion to the ancestral condition brought 

 about by the favourable conditions of the environment. The 

 development of trifoliate leaves may, indeed, be favoured by many 

 circumstances. 



Thus, exposure to light favours the production of leaves with a 

 larger surface, and this in the case of Ulex^ owing to the ancestral 

 tendency, is brought about more readily by the formation of 

 trifoliate leaves than by an increase in the size of the simple leaf. 

 At the same time, the stem is reduced in length and the leaves 

 appear near together at its apex in the form of a rosette, as occurs 

 in many plants grown in exposed situations. This increase in the 

 size of the leaves by exposure to light takes place only to a certain 

 extent, however ; drought tends to reduce them, so that we should 

 expect to find trifoliate seedlings more abundant in places exposed 

 to light, and with sufficient supply of moisture than in those places 

 where one condition is present without the other. Again, it is 

 probable that the reduction of the leaf-surface is in part brought 

 about by cold, which prevents the due absorption of water by the 

 roots, and consequent necessity for economy of the supply already 

 obtained. And as humus soils are warmer than loamy soils, we 

 should expect to find, what is actually the case, that trifoliate 

 seedlings are more abundant on the former than on the latter. 



