MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. 



385 



PIG. 277. 



Pollen-grains of, A, Althcearosea; B, Cobcea 

 mi i scandens; c, Plassiflora ccerulea; D, Ipomcea 



I lie mOSt purpurea. 



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 conditions. By treating 

 some pollen-grains, as those of L ilium Japonicum, L. rubrum, or L. 

 auratum, with the viscid liquid abundantly secreted by the stigma, not 

 only may the extrusion and length- 

 ening of the pollen-tubes be watched, 

 but the grains with their extruded 

 tubes may be preserved almost un- 

 changed by mounting in this liquid. 



388. The darker kinds of pollen 

 may be generally rendered transpar- 

 ent by mounting in Canada balsam; 

 or, if it be desired to avoid the use 

 of heat, ' in the Benzole solution of 

 Canada balsam ( 205), setting aside 

 the slide for a time in a warm place. 

 For the less opaque pollens, the 

 Dammar solution ( 168, d) is pre- 

 ferable. The more delicate pollens, 

 however, become too transparent in 

 either of these media: and it is con- 

 sequently preferable to mount them 

 either dry or (if they will bear it with- 

 out rupturing) in fluid. The most 

 interesting forms are found, for the 



most part, in plants of the orders Amarantacecv, Cichoracece, Cucurbit- 

 acece, Malvaceae, and Passifiorecv; others are furnished also by Convolvulus, 

 Campanula, 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 sylvestris) or of the Hollyhock (AWicea 

 rosea); the anthers being picked soon after they have opened, whilst a 

 large proportion of their pollen is yet undischarged; and being 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 upon a black surface. They are then, when properly illumin- 

 ated, most beautiful objects for objectives of 2-3ds, 1. 1J, or 2 in. focus, 

 especially with the Binocular Microscope. 1 



389. The structure and development of the ovules tnau are produced 

 within the ovarium at the base of the pistil, and the operation in which 

 their fertilization essentially consists, are subject of investigation which 

 have a peculiar interest for scientific Botanists, but which, inconsequence 

 of the special difficulties that attend the inquiry, are not commonly re- 

 garded as within the province of ordinary Microscopists. Some general 



1 It sometimes happens that when the pollen of Pines or Firs is set free, large 

 quantities of it are carried by the wind to a great distance from the woods and 

 plantations in which it has been produced, and are deposited as a fine yellow dust, 

 so strongly resembling Sulphur as to be easily mistaken for it. This (supposed) 

 general diffusion of sulphur (such as occurred in the neighborhood of Windsor in 

 1879) has frightened ignorant rustics into the belief that the ' end of the world ' was 

 at hand. Its true nature is at once revealed by placing a few grains of it under 

 the Microscope. 

 25 



