LIFE HISTORIES OF FAMILIAR PLANTS 



gone even farther and now produce flowers of one 

 sex only on each tree, leaving to insect agency 

 the conveyance of pollen from the male tree to 

 the stigmas of the female. So highly is cross- 

 pollination valued in plant life. 



In regard to the insects also, this evolution 

 presents some fascinating sidelights. In Fig. i8 

 (Plate 13) are shown a few of the hairs on the 

 body of one of the bees seen in Fig. 15 (Plate 1 1), 

 magnified about 350 diameters, and it is interest- 

 ing to notice the pollen grains adhering to them, 

 assisted by delicate branches from the main 

 hairs ; for these hairs are not simple hairs, but 

 hairs specialised to conveniently carry pollen from 

 flower to flower. It need scarcely be said that if 

 the plant evolves devices that assist the bee, the 

 bee will evolve measures of a reciprocal character. 

 This is just a hint of the many interesting aspects 

 of this fascinating subject. 



But we have not yet discovered how our 

 sycamore "key" is produced. In Fig. 19 (Plate 

 12) may be seen the "keys" in the process of 

 manufacture. You will notice that at the end of 

 the lowermost shoot are some male flowers (/.<?., 

 bearing stamens only), but higher up on the same 

 shoot there are others in which, if you look 

 closely, it will be seen that the part bearing the 

 stigma has shrivelled up and the ovary at the base 

 has enlarged into a two-sided winged object. In 

 fact, when they grow a little larger, each one of 

 these flowers will form a pair of sycamore " keys." 

 The process of change is more apparent in the 



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