594 DEFINITION AND CLASSIFICATION OF LEAVES. 



When a botanist of the 16th or I7th century used the word "leaf" in describing 

 plants, it was exclusively in the sense of the popular acceptation of that term. He 

 understood by " leaf ", a flattened outspread structure, such as appears on the 

 branches of trees as a foliage-leaf, green in colour, or, as a floral-leaf, adorned with 

 red, blue, and other colours. Not until the 18th century, and in great part through 

 the influence of Goethe's Essay on Metamorphosis (c/. p. 10), did botanists apply the 

 term " leaf " also to the thick fleshy scales of bulbs, to the scales of winter-buds, to 

 many spines and tendrils, to stamens, and to parts of the fruit-capsule. The causes 

 of the movement in this direction were threefold. First, the wish to collect the 

 extremely manifold phenomena sjmoptically ; the struggle to find a simple general 

 law of nature to which the shapes of innumerable single living organisms would 

 conform; further, the similarity of origin — the agreement actually observed over 

 and over again in the earliest stages of development of structures which afterwards 

 become so different; and, finally, the circumstance that occasionally under abnormal 

 external influences, viz., under the influence of mites, plant-lice, and other animals, 

 green leaves are actually formed from the spines, tendrils, stamens, and fruit- 

 capsules. Now, an original or fundamental type of leaf was imagined, of which 

 naturally the shape of the ordinary green foliage-leaf became a standard of com- 

 parison. It was represented that the other structures enumerated, which do not 

 agree in their shape, although they agree in their origin with the green leaves, 

 had been produced from these by modification, and that they also must be regarded 

 as leaves, of course as changed or metaTnorphosed leaves. According to this view, 

 the bulb-scales, the stamens, and the parts of the fruit-capsule are metamorphosed 

 leaves, although they do not correspond in their adult form to the idea of a leaf 

 conceived by people who are not botanists. 



The struggle after perfection, the gradual refinement of the sap conveyed to the 

 leaves in their first stages, and many other things were at first supposed to be the 

 causes of the transformations. In modern times this metamorphosis is associated 

 with the division of labour, and with the change of function in the members of the 

 plant-body. The green foliage-leaves eflPect the formation of organic materials 

 from inorganic food in sunlight, but they are not suited at the same time for 

 the protection of seeds or for the manufacture of pollen; nor would they be well 

 adapted as underground storehouses for reserve materials. Consequently certain 

 other leaves of the plant assume shapes better suited to these functions, or, in 

 other words, they are metamorphosed to suit the particular function required of 

 them. We, therefore, do not see green leaves, but stamens developed for the 

 manufacture of the pollen; we do not have green, flattened, outspread foliage as a 

 storehouse for reserve materials in the dark bosom of the earth, but thick, white, 

 fleshy scales. The stamens manufacturing the pollen, the green leaf-blades 

 preparing organic materials in sunlight, and organs, of one and the same plant, 

 fitted to various other definite tasks, are so entirely similar in their origin and first 

 stages, of development, that they are included under a common abstraction, and the 

 word " leaf " has been employed to express it. As in a beehive the adult workers, 



