734 THE POPULAR SCIENCE MONTHLY. 



the extreme theoretical and biological point of view, must be re- 

 garded to some extent as a distinct individual. ISTevertlieless, the 

 entire colony of leaves, the herb, shrub, or tree, as we generally 

 call it, has also a sort of complex individuality of its own ; it is 

 an organism in itself, containing various parts or members, such 

 as roots, stems, leaves, and so forth, each performing distinct 

 functions for the good of the entire complex body. Now, the 

 plant goes on for a certain length of time producing leaves from 

 its surplus material one after another ; and, as long as fresh ma- 

 terial is suj^plied, this production or growth seems in some cases 

 to have hardly any distinct limit. Strawberries, for example, will 

 go on sending out runners (which are merely branches with tufts 

 of leaves at the end that root from time to time) almost endlessly. 

 We have here an example of continuous non-sexual reproduction. 

 There are in nature innumerable variations in the manner of such 

 purely vegetative growth. Sometimes, as in deciduous trees, the 

 leaves all fall off in autumn, and totally new ones are brought 

 forth from buds in the succeeding season ; sometimes, as in the 

 potato, new shoots spring from swollen underground branches ; 

 sometimes, as in the crocus, small bulbs are developed as stocks 

 on the top of the old one. But, whatever the variation, the cen- 

 tral fact still remains the same : the leaves, stems, or branches 

 thus put forth are, strictly speaking, parts of the same compound 

 organism, asexually produced, not entirely new and separate indi- 

 viduals. 



A plant, however, no matter how vivacious, can hardly go on 

 living forever. Sooner or later, there is reason to believe, this 

 purely vegetative growth fails. The original vigor of its consti- 

 tution gets used up ; the life and go of the plant become hope- 

 lessly weakened. This seems to be the case at the present day, 

 for instance, with the cultivated potato, which has been propa- 

 gated from the tubers almost exclusively for many years, so that 

 the existing plants must be of immense age, and have grown effete 

 and feeble for want of proper sexual renewal. How are plants 

 which have thus reached their dotage to restore their youth ? 

 How are they to carry on to future years the life of the species ? 



Nature has answered this problem of life by the wonderful 

 device of intercrossing. The organism, like every other machine, 

 tends in time to wear out and decay. But, unlike other machines, 

 it contains in itself (through the action of natural selection) the 

 means for manufacturing its own successors. 



The leaf, we saw, grows out from the leaf. If you cut a piece 

 of the common cactus or prickly-pear, and drop it on the ground, 

 it roots at once and grows up afresh into a full-grown cactus- 

 plant. There are some leaves which, if hung up, produce other 

 leaves and little plants from their edges ; and everybody mv st 



