TETJE NATURE OP PLOKAL ORGANS. 



155 



charged and how it is carried from flower to flower. The 

 commonest method is to have the anther-cells split length- 

 wise, as in Fig. 138, I. A few anthers open by trap-doors 

 like valves, as in II, and a larger number by little holes at 

 the top, as in III. 



The pollen, in many plants with inconspicuous flowers, as 

 the evergreen cone-bearing trees, the grasses, rushes, and 

 sedges, is a fine, dry powder. In plants with showy flowers 

 it is often somewhat sticky or pasty. The forms of pollen- 

 grains are extremely various. That of the tulip (Fig. 116), 

 and the kinds shown in Fig. 

 139 will serve as examples of 

 some of the shapes which the 

 grains assume ; IV in the latter 

 figure is perhaps as common a 

 form as any. Each pollen-grain 

 consists mainly of a single cell, 

 and is covered by a moderately 

 thick outer wall and a thin inner 

 one. Its contents is a thickish 

 protoplasm, full of little opaque 

 particles and usually containing 

 grains of starch and little drops 

 of oil. The larger knobs on the 

 outer coat, as at k (Fig. 139, I 

 and II), mark the spots at which 

 the inner coat of the grain is 

 finally to burst through the outer 

 one, pushing its way out in the form of a slender, thin-walled 

 tube.i ' 



190. Exp erim ent32. Production of Pollen Tubes. — Place a few 

 drops of suitably diluted syrup ^ with some fresh pollen in a concave cell 

 ground in a microscope slide ; cover with thin glass circle ; place under 



1 See Kerner and Oliver, vol. II, pp. 95-104. ' See Appendix B. 



Fig. 139. — Pollen-Grains. 

 I, hazel ; II, coltsfoot ; III, wild 

 ginger ; IV, hepatioa ; V, pine ; 

 s s, air-sacs, 

 diameters.) 



(All magnified 300 



