POLLEN-GKAINS 



721 



away. Sometimes the pores are covered by little disc-like pieces or 

 lids, which fall off when the pollen-tube is protruded. This action 

 takes place naturally when the pollen-grains fall upon the surface of 

 the stigma, which is moistened with a viscid secretion ; and the 

 pollen-tubes, at first mere protrusions of the inner coat of their cell, 

 insinuating themselves between the loosely packed cells of the stigma, 

 grow downwards through the style, sometimes even to the length of 

 several inches, until they reach the ovary. The first change, namely 

 the protrusion of the inner mem bi^ine through the pores of the exterior, 

 may be made to take place artificially by moistening the pollen 

 with water, thin syrup, or dilute acids (different kinds of pollen- 

 grains requiring different modes of treatment) ; but the subsequent 

 extension by growth will only take place under the natural con- 

 ditions. By treating some pollen -grains, as those of Lilium 

 japonicum, L. rubrum, or L. auratum, with the viscid liquid abun- 

 dantly secreted by the stigma, 

 not only may the extrusion 

 and lengthening of the pollen - 

 tubes be watched, but the 

 grains with their extruded 

 tubes may be preserved almost 

 unchanged by mounting in 

 this liquid. 



The darker kinds of pollen 

 may be generally rendered 

 transparent by mounting in 

 Canada balsam ; or, if it be 

 desired to avoid the use of 

 heat, in the benzol solution 

 of Canada balsam, setting- 

 aside the slide for a time in 

 a warm place. For the less 

 opaque pollens the dammar 

 solution is preferable. The 

 more delicate pollens, how- 

 ever, become too transparent 



in either of these media ; and it is consequently preferable to mount 

 them either dry, or (if they will bear it without rupturing) in fluid. 

 The most interesting forms are found, for the most part, in plants 

 of the orders Amaranthacew, Gichoriacece, Cucurbitacece, Malvacece, 

 and PassiflwecK ; others are furnished also by Convolvulus, Cam- 

 panula, CEnothera, Pelargonium (geranium), Polygonum, Sedum, and 

 many other plants. It is frequently preferable to lay down the 

 entire anther, with its adherent pollen-grains (where these are of 

 a kind that hold to it), as an opaque object ; this may be done with 

 great advantage in the case of the common mallow (Malva syl- 

 vestris) or of the hollyhock (Althcm rosea), the anthers being picked 

 soon after they have opened, whilst a large proportion of their pollen 

 is yet undischarged, and laid down as flat as possible, before 

 they have begun to wither, between two pieces of smooth blotting- 

 paper, then subjected to moderate pressure, and finally mounted 



FIG. 565. Pollen-grains of A, Altltcea rosea 

 (hollyhock) ; B, Cobcea scandens ; C, Passi- 

 flora ccerulea; D, Ipomcea puryurea. 



