CHAPTERS ON- EVOLUTION. 



duction. Sooner or later, the anthers of the stamens open in one 

 way or another so as to allow the pollen to escape ; and, viewed under 

 the microscope, the pollen-grains are seen to vary greatly in size and 

 form in different species of plants. The grains of pollen may be 

 round (Fig. 220) "or oval in form ; in the evening primrose (Fig. 222) 

 and fuchsia, they are of triangular shape ; in the hollyhock and 

 melon (Fig. 223) they are spinous ; and in the orchids they are united 

 to form masses (Fig. 221) called pollinia. 



The pollen-grains being conveyed to the stigma (Fig. 218, sg) of the 

 pistil, they are there attached by the aid of a glutinous secretion, which 



FIG. 220. POLLEN-GRAINS EMITTING 

 POLLEN-TUBES. 



FIG. 223. POLLEN- 

 GRAIN OF MELON 

 EMITTING CONTENTS. 



FlG. 222. 



POLLEN-GRAIN OP 



EVENING PRIMROSE 



(MAGNIFIED). 



may likewise be credited with a specific influence on the pollen- 

 grains, in that it appears to stimulate the curious development they 

 next evince. This development consists in the rupture of the outer 

 of the two layers of which each pollen-grain consists. Through the 

 ruptured outer coat, the inner layer begins to grow in the form of a 

 long tube the pollen-tube (Fig. 220) which penetrates the tissue of 

 the style (Fig. 224), and grows downwards to reach the ovules con- 

 tained in the ovary. In some plants, the pollen-tubes emitted from 



