RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 413 



single leaf had been plucked from the lower or upper part of the stem. Only the 

 lowest leaves of all, whose shadow falls on the ground and not on neighbouring 

 leaves, are divided into broader sections; the others are equally divided and project 

 at equal intervals round the stem. While the Mullein, with its entire foliage- 

 leaves, rapidly diminishing in size towards the summit, presents a pyramidal 

 appearance from a distance; the Fennel and Larkspur, whose finely-divided leaves 

 are similar all along the stem, rise up like cylindrical columns. In other words, if 

 the extreme outer point of all the leaves of the last-named plants are connected 

 together in a surface, this will take the form of a cylinder. Only when projecting, 

 divided leaves are crowded above one another on a very short stem, as, for 

 example, in ferns, and where the plants are growing in shady places where the 

 light is very scanty, it happens that the lower leaves are raised above the upper 

 in order not to miss too much of the enjoyment of the light. 



The perforation of the leaf-blades, which is observed, though but seldom, in 

 many aroids, has now to be considered. The best known in this respect are the 

 Brazilian Monstera egregia, and the Tornelia fragrans, illustrated in fig. 96^ 

 which has also been called by gardeners, in consequence of the gaps in the leaves, 

 Philodendron pertusum. The circular or elliptical holes do not originate late 

 on in the leaf-blade, but can actually be seen when the small and undeveloped 

 leaves are yet folded. They are always formed on the upper leaves of older 

 plants; the leaves of younger, shorter specimens do not possess them. This 

 circumstance suggests that the holes have the same significance as that previously 

 assigned to the deep incisions and clefts between the leaf -lobes. They are chinks 

 in the broad upper leaves whose shadow extends over a large area, through 

 which a portion of the obliquely falling rays of light can reach the more deeply 

 situated leaves. The peculiar notches in the blades of certain leaves of the Black 

 Mulberry -tree (Morus nigra), as well as of the Japanese Paper Mulberry 

 (Broussonetia papyrifera), may be explained in like manner. They are only 

 found on the upper leaves of a branch, and are best seen on the erect slender 

 shoots which spring from the base of old trunks. Sometimes, in these highest 

 leaves, only one half has an incision extending almost as far as the midrib; some- 

 times again both halves are provided with deep clefts; in the highest shoot- 

 leaves of the Black Mulberry-tree the blade is often divided up into fairly narrow 

 segments by several incisions on both sides. If such developing shoots, crowded 

 closely together, are observed at mid-day, when they are directly illumined by 

 the sun, the shadow of the upper leaves can be seen sketched out on the leaves 

 below, but to each incision and indentation of a leaf at the apex, a patch of light 

 corresponds on the leaf-surfaces in the stories next below. Suppose now that 

 the holes above had been closed; immediately it would become darkened under- 

 neath, the spots of light which continually move according to the position of 

 the sun from place to place and from leaf to leaf would be wanting, and the 

 activity of the green tissue in the leaves of the lower region would be to some 

 extent impaired. 



