The Profound Effect on the Structure of Plants 55 



there is, without any shading by its neighbors. This ideal ex- 

 posure allows the development of the ideal type of construction, 

 i. e., the shape that encompasses the most green tissue within 

 the least outline, and a venation ensuring the shortest paths for 

 conduction of water and the photosynthate. Such a leaf must be 

 round, with its veins radiating from a central petiole. It is well- 

 nigh realized in the leaf of the Common Garden Nasturtium 

 (figure 11, c), a low-stemmed plant whose long petioles permit 

 a full exposure of each leaf to light (figure 12) ; and it is shown con- 

 ventionalized in figure 13, a. Furthermore, this association of 

 round-radiate (or, in the current terminology, round-palmate), 

 shape with full exposure to light is actually found in most plants 

 which grow in such manner that their leaves do not shade one 

 another, as for example in the floating leaves of Water Lilies 

 (figure 11, a), Ground Ivy (figure 11, 6), Wild Ginger, and others 

 which trail or creep on the ground, and in low-growing long- 

 petioled herbs like Geranium, Cyclamen, and Pelargonium, and 

 partially in Ivies. Most of these leaves show a slit from the 

 petiole to margin, but that does not alter the principle of the 

 central-standing petiole, for the slit is merely a relic of the evolu- 

 tion of these leaves from kinds in which the petiole stood on the 

 margin; indeed all intermediate gradations exist in heart-shaped, 

 arrow-shaped, and "auriculate" leaves, where a part of the blade 

 bulges backward on each side of the petiole. 



Second, the opposite extreme of habit is found where leaves 

 are compelled to grow crowded together, as they are in most plants 

 living in especially dry or light places. In this case the best shape 

 and arrangement would be necessarily the exact opposite of those 

 found in the round type, that is, the leaves would be slender or 

 linear, without distinction of petiole and blade, and with the veins, 

 running parallel; while they would take such positions as would 

 admit the light most deeply and evenly among them, viz., they 

 would point at the light and therefore stand parallel or radiating 

 with respect to one another. Such a position for the leaves is in 



