ORGANS OF FRUCTIFICATION 



69 



grubis. Sometimes it is in solid masses of greater 

 ui' less size ; but as, in this state, it occurs in 

 only a few plants, we shall first examine the pol- 

 len in the powdery form. 



Previous to the improvement of optical instru- 

 ments, the knowledge which had been obtained 

 respecting the varied forms of the grains of pol- 

 len, and especially respecting their internal struc- 

 ture, was extremely vague. A great diversity 

 had indeed been perceived in those which had 

 been examined with powerful lenses, but their 

 differences had been pointed out without deriv- 

 ing fi-om them any references that might tend 

 to the advancement of science. The structure 

 of the pollen had also engaged the attention of 

 most of the botanists, who had long disputed, 

 without coming to any settled determination, res- 

 pecting the internal composition of bodies of so 

 elementary a nature. The microscopic examina- 

 tion of the pollen was therefore a subject that re- 

 quired revision, and which could not fail to 

 attract the attention of modern observers. The 

 grains of the pollen are utricles of various forms, 

 having no adhesion to the anther at the period 

 of maturity, and containing a multitude of gran- 

 ules of extreme minuteness. The utricular 

 membrane is sometimes smooth, sometimes 

 marked with eminences or asperities. Some- 

 times it presents little flat surfaces or promin- 

 ences symmetrically arranged. When the pol- 

 len is perfectly smooth at its surface, it is not at 

 the same time covered with any viscous coating, 

 whereas the slightest eminences are indications 

 of this adhesive covering. The papillae, mam- 

 millary eminences, &c., which cover certain 

 grains of pollen, are true secreting organs, of 

 which the viscous and usually coloured envelope 

 with which they are invested is the product. 

 'J'he powdery pollens may therefore be arranged 

 under two principal ordera, the viscous and the 

 non-viscous, pollens. 



M. Guillemin discovered, by extensive obser- 

 vation, that the nature of the grains of pollen 

 is the same in each natural family of plants ; or, 

 in other words, that viscous and non-viscous 

 pollens never occur together in the same family. 

 He has found, moreover, that all the genera of a 

 family present only modifications in the forms 

 of their grains of pollen ; although families very 

 remote from each other in reSpect to other char- 

 acters, agree in having the same kinds of pollen. 

 "We shall here content ourselves with describing 

 the nature and forms of this organ in a few re- 

 markable families. 



The pollen of the Mallow and Convolvulus 

 families is fonned of papillar spherical grains, 

 of a silvery white colour. In the cucumber, 

 they are spherical, papillar, and of a beautiful 

 gold-yellow. Those of the tribe of helianthece, 

 in the family of synantliereoe, are also spherical, 

 pnpillar, and of a fine orange-yellow. The tribe, 



or rather order, of the ciclioracew, presents spheri- 

 cal grains, which are viscous, but are bounded by 

 minute plain surfaces. In colwa scandens, the 

 pollen is covered with mammillar eminences, 

 each surmounted by a shining point. The pollen 

 of the genus phlox very much resembles that 

 mentioned last ; and this is a circumstance corro- 

 borative of the opinion of those who consider the 

 two genera as belonging to the same natunJ 

 family. 



The families in which grains that are not 

 viscid are found are very numerous. As in the 

 potatoe, gentian, grasses; and the grains in 

 these have always an elliptical form, and are 

 marked with a longitudinal groove. Their usual 

 colour is yellow, although they are sometimes 

 red, as in verbascum. In the pea tribe, the pol- 

 len, although not viscous, is of a very distinct 

 cylindrical form. 



When grains of pollen which ai'e not viscous are 

 subjected to the action of water, they instantly 

 change their fonn, which, from being ellipti- 

 cal, becomes perfectly spherical. The viscous 

 grains first lose their coating, then burst more 

 or less quickly, and project a fluid denser than 

 water, and in which are seen moving myriads of 

 minute grains, which are rendered visible by 

 their greenish colour, when they are magnified 

 to several hundred diameters. Amici saw a 

 grain of pollen, in contact with a hair of the 

 stigma, burst, and project a kind of bowel, in 

 which the minute grains circulated for more 

 than four hours. Gleichen, who had already 

 observed the granules contained in the grains of 

 pollen, considered them as performing the prin- 

 cipal part in the act of fecundation ; and Guil- 

 lemin, reasoning from the resemblance of these 

 organs to the spennatic animalcules of animals, 

 is inclined to adopt the same opinion. 



Such was the state of our knowledge respect- 

 ing the nature and organization of the grains of 

 the pollen, when Brongniart undertook his ex- 

 amination of the generation of vegetiibles. His 

 opinion respecting the nature and organization 

 of the grains of pollen is as follows : — On ex- 

 amining the interior of the cells of a yellow 

 anther in a flower-bud, long before its expansion, 

 it is seen to be filled with a cellular mass distinct 

 from the walls of the cells. By degrees the 

 cellules of which the cellular mass is composed, 

 and which are generally very small, separate 

 from each other, and at length form the gran- 

 ules, which are named pollen. Sometimes these 

 particular cellules or gi'ains of pollen are enclosed 

 in other larger vesicles, which become torn, am; 

 of which traces may still be perceived. 



Each grain of pollen, whose form, as has al- 

 ready been remarked, is very variable, presents 

 a uniform organization. It is composed of two 

 membranes, the one external, tliicker, and fur- 

 nished with pores, and sometimes more or Itss 



