Ill 



microscope they are seen to serve also the purpose of keeping 

 properh' distributed a relatively v^ery thick layer of viscous semi- 

 fluid oil with which these grains are provided. This is a matter 

 of minute contours in relation to surface tension and finds its 

 analogy in the fact that you must roughen with sandpaper a 

 newly varnished surface before applying the second coat, or the 

 new coat, while still fluid, will gather into droplets instead of 

 remaining smooth and even. It is this enveloping layer of oil 

 which serves to stick the grains to each other and to the insects 

 which are to carry them; pollen grains lack the weight to make 

 their spines or other sculpturing effective in preventing skidding. 



Pollen grains are nearly always very susceptible to changes in 

 moisture. So rapidly do they absorb moisture from their sur- 

 roundings that it is difficult to accurately weigh a gram or two 

 of most pollen in ordinary air. As the grains take up moisture 

 they expand consequently they must be provided with some 

 mechanism for accommodating such volume changes without 

 prematurely rupturing the non-distensible outer coat. In the 

 goldenrod pollen grain volume changes are accommodated by 

 three meridionally arranged furrows. When such a grain is dry it 

 is ellipsoidal in form with three deep longitudinal grooves, the 

 delicate inner coat completely covered by the protective outer 

 coat. This is the resting condition of the pollen grain. But as 

 soon as it is quickened by the touch of a little moisture the fur- 

 rows open, very much as the eye opens after sleep, and each 

 furrow is seen to be crossed by a delicate membrane like the 

 cornea of the eye, and in place of the pupil is a central open- 

 ing, the germ pore, through which may protrude the germinal 

 papilla, accounting for still further internal expansion of the 

 grain. As the furrows expand the grain changes its shape from 

 elongate ellipsoid to spheroid thereby greatly increasing its 

 volume without materially affecting its linear dimensions. 



The three-furrowed type of grain is by far the commonest 

 among the higher dicotyledons, accounting for about two-thirds 

 of all species, and may be regarded as the basic form from which 

 all others are derived. The sculpturing varies with the genus, 

 tribe or family. In most of the tribes of the composite family it 

 is echinate like the goldenrod grain, but in the chicory tribe the 

 surface is thrown into an elaborate system of ridges with the 

 spines on their crests, as in the grains of dandelion. In the 



