THE POLLEN-GRAIN. 387 



Evening Primrose) which is ready to expand, we shall find 

 the anthers have already dehisced and set free their pollen- 

 .rains. These latter are suspended between the anthers by 

 obweb-like threads. If such threads are stretched out upon an 

 >bject-slide, they appear under the microscope as exceedingly deli- 

 ;ate filaments, some stretched straight, others tangled. The 

 x>llen-grains in a dry state are opaque, but their triangular form 

 once noticeable. In water, with a higher power, they show 

 Uttened, sj-mmetrically-triangular bodies, with rounded pro- 

 jecting angles. At the base of each of these rounded corners 

 in inner annular thickening of the membrane of the pollen-grain 

 is to be seen. The contents of the pollen-grain appear finely 

 granular ; the two nuclei are only recognisable in the contents 

 of the ripe grain with extreme difficulty. In sulphuric acid the 

 exine assumes a red-brown colour. By it an outer, thin, yellow- 

 coloured layer is lifted up, forming folds upon an inner, thicker, 

 red-brown layer. Both layers coalesce in the uncoloured wall 

 of the rounded corners. From the side walls of the corners 

 fine teeth project inwards. The apex of the corners is dissolved 

 by the sulphuric* acid. The fine filaments, binding the pollen- 

 grains together, resist water, sulphuric acid and potash, and are 

 insoluble also in alcohol. If the grams are treated with 25 per 

 cent, chromic acid, then* membrane is soon dissolved, and in all 

 cases the strongly-cuticularised portion somewhat sooner than 

 that which is either not, or but slightly cuticularised, which 

 remains for a time as colourless swollen caps covering the rounded 

 angles. Later on, these also are dissolved ; and ultimately even 

 the cobweb threads between the grains. 



Development of the Pollen-Tube. From the stigma of an older 

 flower of (Enothera pollen-grains can be lifted off which have 

 already germinated and developed pollen-tubes. In this case 

 also there is an inner colourless intine surrounding the contents 

 of the pollen-grain, which develops into the pollen-tube through 

 one or other of the angles. The formation of a pollen-tube takes 

 place usually at one angle only, that one which happens to be 

 most favourably placed ; or if more than one tube is started, only 

 one continues to develop. Instead of (Enothera, an Epilobium 

 ( Willow-herb), or a Fuchsia can be used for study. 



Longitudinal sections of the arms of the style of flowers of 

 f Enothera, etc., which are past their prime, will show the pollen- 

 tubes, developed from the corners of the pollen-grains, traversing 



