1867.] 315 [Agassiz. 



outer halves repeating each other in opposite dii^ections, and their 

 inner halves in like manner, just as is the case with our two eyes. 

 The accompanying figures show the above mentioned differences. 



In the elm leaves the veins of the outer side are, of course, in ad- 

 A'ance of the corresponding veins on the inner and larger side ; and 

 in the Ostrya the reverse is to be seen, though less readily, each vein 

 of the inner side being (at least in the basal portion of the leaf,) a 

 little in advance of the corresponding vein on the opposite side of the 

 midrib. And in both kinds of leases the difference is sufficiently 

 plain to enable one to distinguish them as "rights and lefts," even 

 when separated from the stem. 



Some other leaves, as, for instance, those of the Begonias, have al- 

 ready been noticed to present even more marked differences in the 

 size of the two halves, and there have been offered two explanations 

 of the fact. 



De Candolle thought that it was due to the position of a leaf upon 

 the stem, and that, therefore, the lower or outer side was always the 

 larger: that this explanation is not sufficient is shown by the fact 

 that all elm leaves have the inner or upper side much the larger. 



Herbert Spencer believes that it is caused by the shading of one, or 

 the other half of each leaf, by the leaf above, and that the half so 

 shaded from the sun is less developed than the other : to this one may 

 object, that the same diflei'cnce in the size of the two sides exists, 

 when, from the peculiar position, or smallness of its neighbors, a leaf 

 is not shaded at all; and also the same difference is found on examining 

 the embryo leaf in the bud long before either heat or gravitation 

 could have any effect. 



Spencer being a believer in "variation through natural selection," 

 would meet these objections with the doctrine that, although the 

 leaves of the first elms which grew may have been symmetrical, yet 

 that, in successive generations, the effect of the shading had become 

 so established as to be transmitted, and thus exist independently of 

 the original cause. 



I believe, however, that such peculiarities are true and original 

 characteristics of the plants, and that they are produced by the 

 so-called vital force acting in a definite way, and independently of 

 either heat or o-ravitation. 



Professor Agassiz remarked that the German botanists, 

 and especially Schimper and Braun, had long since investi- 

 gated the develojjment of leaves in connection with the gen- 

 eral subject of phyllotaxis. They had found that each leaf 

 was primarily a swelling or wave of growth, freeing itself 

 from the axis of the embryo, and that differences in size be- 



