Evolution of Veyetal Life. 125 



sionally observed, in which perfect seeds appear to have 

 been formed without fertilization. 



I have said that the leaf is the essential organ of a per- 

 fect plant. Many plants consist only of the leaf. Rising 

 above this stage, and reaching that of a stem, with root and 

 lateral appendages, we still find the leaf the most important 

 feature, and upon it is dependent the character of the plant. 

 The leaves appear upon the stem in certain specific relations. 

 They are opposite, or in a whorl around the stem, or with 

 the stem passing through them, or placed like a shield upon 

 its summit, or arranged alternately upon its sides. If alter- 

 nate, they are arranged in certain spirals, as, in one turn 

 with two leaves, or one turn with three leaves, or two turns 

 with five leaves, or three turns with eight leaves, or five 

 turns with thirteen leaves, etc., before a point is reached ex- 

 actly above that of starting. Sometimes, instead of their 

 ordinary form, leaves assume those of bracts, of scales, as 

 in buds, of tendrils, of spines, etc., and, as we have already 

 seen, of the various parts of flowers. In flowers they exhib- 

 it almost every conceivable variety of form and color : some- 

 times there is one, sometimes there are two floral envelopes, 

 their leaflets united or separate, few or many 5 sometimes 

 there are stamens and pistils ; sometimes either ; sometimes 

 none ; sometimes the flower is regular in shape ; sometimes 

 irregular, in one or other direction, — any one of the parts 

 occasionally reverting to its normal form as a simple leaf. 

 As the branches usually appear at the axils of the leaves, 

 the arrangement of the branches is governed by that of the 

 leaves. 



I have said that plants differ in structure in every respect 

 except one. That one is the cell. As the individual plant 

 starts with a single cell, and, simply by aggregation of cells 

 growing from this one, obtains at last all its varied parts as 

 a perfect whole, so the vegetable kingdom throughout, from 

 the simplest form to the most complex, is but a series of 

 similar aggregations. Under the development hypothesis it 

 is claimed that these forms are of a common stock ; are re- 

 lated to each other by lines of descent, all having probably 

 originated in the unicellular * aggregation of protoplasm 

 which I have described. As we cannot say how this became 

 differentiated from inorganic matter, so we cannot positive- 

 ly say whether such differentiation can now take place. 

 The problem of spontaneous generation is one to which 



