122 RELATIONSHIPS OF SYMMETRY 



leaflet ; they are only half as thick, and they have either no evident 

 palisade-cells, or only a series of quite short palisade-like cells, whilst in 

 the terminal leaflet there are two rows of palisade-cells. In other words, 

 their structure reminds one of that in shaded leaves, and this gives 

 support to the view that the reduction or suppression is a result of the 

 hinder side of the leaf occupying a position in which it receives less 

 light than the fore side, chiefly because it is covered by the sides of the 

 axillary shoots. 



b. ASYMMETRY AND UNEQUAL SIZE OF LEAFLETS. 



We have here to speak of two kinds of relationships (a) firstly of 

 the asymmetric construction which appears in many leaflets, and (/;) 

 secondly of the unequal construction of the single leaflets of a compound 

 leaf which docs not cause asymmetry of the whole leaf. 



a. ASYMMETRY OF LEAFLKTS. 



We frequently observe in compound leaves that the terminal 

 leaflet is symmetric whilst the lateral leaflets are asymmetric. From 

 the large number of examples I select the following: species of Rubus, 

 species of Tetragonolobus and Phaseolus amongst Leguminosae, Juglans, 

 Chelidonium, Fraxinus, Heracleum plants it will be noted of the most 

 different cycles of affinity. Using the term 'higher' for that side of 

 a leaflet which is directed to the point of the leaf we find that it is 

 the lower side, seldom the higher, which is the larger ; see, for example, 

 Cedrela amara, Caesalpinia Sappan, Tamarindus indica plants which 

 I mention here because I have them before me in a living condition. 

 De Candolle 1 has expressly stated that in asymmetric leaflets the lower 

 side is always the larger, but what I have said shows that this is not 

 quite correct. Hofmeister has endeavoured to trace the asymmetry of 

 lateral leaflets to a unilateral influence of gravity. He says 2 : 'The 

 lateral leaflets of many compound leaves both pinnate and digitate show an 

 evident superiority in the outline of the lower side of the lamina over 

 the higher side. If such compound leaves have a terminal leaflet its 

 sides are equally developed. As examples we have Pavia macrostachya, 

 Aesculus Hippocastanum, Ptelea trifoliata, Staphylca trifoliata, Rosa 

 pomifera and R. gallica, Sorbus Aucuparia, Rubus Idaeus and R. fruticosus, 

 Pterocarya caucasica, Robinia viscosa, Cytisus Laburnum, Gleditschia 



1 De Candolle, Organographie Vegetale, i. p. 346. 

 j 1 Hofmeister, Allgemeine Morphologic, p. 592. 



