Senile Changes in Leaves of Certain Plants 293 



determinations of the area of the cell aggregates have here been 

 employed. 



The average size of these aggregates, which will hereafter be called 

 vein islets, was determined by dividing the area of each of the openings 

 in the paper by the number of vein islets that filled it in the photograph. 

 The counting was done under a lens, and a sharp needle was used to 

 prick each vein islet as it was counted on the photograph. 



influence of certain factors on venation 

 While the literature on the venation of leaves is very extensive, most 

 of it is concerned with the development, structure, and function of the 

 large veins — the midrib and its main branches. Only one phase of 

 these investigations is directly important to the present problem, and 

 that is the result of the controversy over the question whether the veins 

 follow a course in the leaf entirely predetermined by heredity, or whether 

 external conditions while the leaf is expanding can exert an influence. 

 The paleontological botanists, as represented by von Buch (1852), 

 went so far as to say that, no matter how much the leaf might expand, 

 no more nerves could arise than those present in the leaf while in the bud. 

 The attack on this idea began with the discovery by vonNageli (1855) that 

 in the leaves of Aralia spinosa the nervature was not completely performed 

 in the bud but arose in part later. Prantl (1883) showed that in many 

 dicotyledons, during the expanding of the leaf, the development of new 

 nerves took place in the direction of the greatest expansion. Deinega 

 (1898), with further evidence obtained from the study of both monocoty- 

 ledons and dicotyledons, established the fact that the nervature of the 

 leaf is not performed in the bud to any great degree. 



It still remained to be proved, however, that external conditions exert 

 an undoubted effect on the nervature as it is laid clown in the expanding 

 leaf. While a long list of investigators have determined the effect of light 

 and shade, dryness and moisture, on various structures of the leaf, practi- 

 cally no studies had been made regarding the effects of these external 

 factors on venation until the work of Schuster (1908). It is true that a 

 paper by von Zalenski (1902) was published, in which it was shown that in 

 plants of the same genus a species inhabiting a dry location has a greater 

 development of the finest nerves than another species inhabiting a wet 

 location. Von Zalenski found that Trifolium lupinaster, which lives in a 



