Twenty- TRIED annual meeting. 171 



ing leaf on the ojiposite side: but it is during this unsymnietrical condition that the 

 investigator finds the key to their manner of division or increase (figs. 25-29). It 

 should be borne in mind, however, that the idea is not meant to be conveyed that 

 this division of leaves takes place during a single season — i. e., it is not meant that 

 the individual leaf or leaflet undergoes this entire evolutionary division during a 

 single season. Various stages of division may be found on one and the same plant 

 at the same time, but only a very slight advance on the previous year is made by 

 succeeding years. Slowly, but surely, heredity transmits this tendency to succeeding 

 generations. So numerous and various are the plants exhibiting this proclivity 

 toward division, that one is led to inquire. Why this tendency in leaves to divide? 



Grant Allen, in his delightful little book on "Flowers and their Pedigrees," says: 

 "Leaves depend for their growth upon air and sunlight: they must be supplied with 

 carbonic dioxide to assimilate, and solar rays to turn off the oxygen and build up 

 the carbon into their system. In open fields or bare spaces, big leaves like burdock 

 or rhubarb can find food and space, but where carbonic dioxide is scarce, and light 

 is intercepted by neighboring plants, all the leaves must needs be fine and divided 

 into almost thread-like segments. The competition for carbon under such circum- 

 stances is exceedingly fierce." For example, he continues, "in water only very small 

 quantities of gas are dissolved, so that all submerged water plants have extremely 

 thin waving filaments instead of flat blades, . . . while hedge-row weeds which 

 jostle thickly against one another have a constant hard struggle for the carbon and 

 sunshine, and grow out accordingly into numerous subdivided leaflets, often split up 

 time after time into segments and sub-segments of the most intricate sort." 



Sir John Lubbock, in his little book on "Flowers, Fruits, and Leaves," offers a 

 somewhat different explanation. Taking the position of the leaves, the direction of 

 the bough and various other elements into consideration, he says "it seems clear that 

 there is a correspondence between thickness of stem and size of leaf. This ratio, 

 moreover, when taken in relation with the other conditions of the problem, has a 

 considerable bearing, not only on the size, but also on the form of the leaf." Lub- 

 bock also accounts for the minute dissection of submerged aqueous plants on the 

 same principle. "We know," to use his own words again, "that the gills of fish con- 

 sist of a number of thin plates which, while in water, float apart, but have not suffi- 

 cient consistence to support even their own weight, much less any external force, 

 and consequently collapse in air. The same thing happens in thin, finely-cut leaves. 

 In still water, they afford the greatest possible extent of surface with the least ex- 

 penditure of effort in the formation of skeleton. The conditions of still air," Lub- 

 bock continues, "would approximate those of water, except so far as they are 

 modified by weight, and the more the plant is exposed to the wind, the more it 

 would require strengthening, hence perhaps the fact," he concludes, "that herbs so 

 much oftener than trees have finely-cut leaves." 



Of the two views presented, those of Grant Allen seem the more plausible. Heat 

 and light are certainly the great forces which impel the vegetable world; in order to 

 grow, plants must have air and sunlight, and it seems clear that both air and sun- 

 light will come in contact with a greater leaf-surface when the leaves are much cut 

 up or divided than when they are large. The air and light would pass more freely 

 about, among and over the smaller leaves, and a greater number would receive a 

 portion of these necessities of nlant-life. 



The element of strength suggested by Lubbock, may also j^lay a part in deter- 

 mining the size and shape of leaves, but it may be viewed from another standpoint 

 perhaps equally plausible. Instead of those plants which are weakest and less ex- 

 posed being divided because they need less frame-work or skeleton, may it not be a 

 method of adaptation which enables them to withstand wind and storm? Small 

 leaves, or those which are much divided, certainly offer less resistance to wind and 



