736 THE POPULAR SCIENCE MONTHLY. 



parts out of its own material. Old age, as we say, comes on. And 

 this is S23ecially true of the higher and more complex organisms. 

 The tree no longer puts forth new leaves ; the plant no longer 

 sends out fresh branches. Its individual vigor appears to be used 

 up. Unless, then, some fresh stimulus can be suj)plied it from 

 without, the plant must die, and the species thus must suffer ex- 

 tinction. 



At this point, therefore, Nature steps in with a special remedy 

 — the special remedy of cross-fertilization. The earliest and sim- 

 plest form of this device is seen in certain algse or pond-weeds, 

 mere long green hairs that wave about like tresses in the water, 

 and consist each of endless rows of cells growing out in single file 

 like the beads of a necklace one from the other. But every now 

 and then two of these algse " conjugate," as biologists put it — that 

 is to say, a cell of one bends over and unites with a cell of the 

 other, the cell-contents (or protoplasm and chlorophyl) of one 

 cell breaking through to join the cell-contents of its neighbor. 

 The union thus effected seems to supply a fresh stimulus to 

 growth : the two matters coalesce and combine, and a new and more 

 vigorous alga springs up as the final result of this combination. 



Now, in the higher plants we get exactly the same sort of com- 

 bination, only far more complex in its mechanism and results. If 

 we take any annual plant, like the pea, and look when and where 

 the flowers are produced, we shall see that they come as soon as 

 the plant has attained its full growth, and when the purely vege- 

 tative reproductive impulse is beginning to fail. As a rule, too, 

 the flowers come at the end of the branches, and in many — indeed, 

 in most — plants they form a terminal spike or bunch at the sum- 

 mit of the stem, as in the familiar instances of the hyacinth, the 

 buttercup, the sunflower, or the grasses. In other words, as soon 

 as the vegetative growth is beginning to slacken, the need is felt 

 for " fresh blood," for the special stimulus or fillip to further ex- 

 ertion given by union with another individual. 



For the yjurpose of bringing about the desired union, all the 

 higher plants are supplied with special organs known as stamens 

 and pistils. The pistils produce the embryo seed, which is, in fact, 

 a tiny separate plant, whose development is arrested at a very 

 early stage, unless fresh material from a neiglibori7ig stamen is 

 supplied to supplement it. The stamens produce the pollen-grains, 

 which are, in fact, free cells containing a large quantity of very 

 vitalized matter capable of fertilizing and vivifying the embryo 

 seed. When a grain of pollen is placed by any agency whatsoever 

 — wind, an insect, or a camel's-hair brush, as the case may be — on 

 a neighboring stigma, it sends out a pollen-tube which penetrates 

 the ovary and at last enters into and coalesces with the embryo 

 seed itself. The fresh material thus added to the embryo seems 



