140 MORPHOLOGICAL DEVELOPMENT 



which is a diagrammatic section down the midrib of the 

 leaf, showing its inclined attitude and the positions of the 

 wings a, b, c, will make the cause clear. As the wings 

 overlap like the bars of a Venetian blind, each intercepts 

 some light from the one below it; and the one below it 

 thus suffers more on its distal side than on its proximal side. 

 Hence the smaller development of the distal side. That this 

 is the cause is further shown by the proportion that is main- 

 tained between the degree of obscuration and the degree of 

 non-development ; for this unlikeness is greater between the 

 two sides a and a', than between b and b', or c and c', at the 

 same time that the interference is greater in the lower wings 

 than in the upper. Of course in this case and in the kindred 

 cases hereafter similarly interpreted, it is not meant that this 

 differentiation is consequent solely, or even chiefly, on 

 the differential actions experienced by the individual plant. 

 Though there is good reason to believe that the rate of growth 

 in each part of each leaf is affected by the incidence of light, 

 yet contrasts so marked and so systematic as these are not 

 explicable without taking into account the inheritance of 

 modifications either functionally caused or caused by spon- 

 taneous variation. Clearly, the tendency will be towards 

 the preservation of a plant which distributes its chlorophyll 

 in the most economical way ; and hence there will always be 

 a gravitation towards a form in which shaded parts of leaves 

 are undeveloped. 



229. From compound leaves to simple ones, we find 

 transitions in leaves of which the divisions are partial in- 

 stead of total ; and in these we see, with equal clearness, the 

 relations between forms and positions that have been traced 

 thus far. Fig. 213 is the leaf of a Winter-aconite, in which, 

 round a vertical petiole, there is a radial distribution of half- 

 sepai'ated leaflets. The Cecropia-lea.f, Fig. 214, shows us a 

 two-sided development of the parts beginning to modify, 

 but not obliterating, the all-sided arrangement ; and tliia 



