THE YEGETABLE CELL. 1S3 



seems always to become more or less granular and opake. The 

 pollen-tubes are distinguishable from the cells of the conducting 

 tissue, partly by their opake contents, and partly by their smaller 

 diameter (which is often very small, e, g., in Orchis Morio about 

 1-1 80th of a millim, in Digitalis purpurea l'-]66th, in Oheir- 

 anthus Cheiri l~280th, in Gapsella Bibrsa-pastoris 1-832). 



Arrived in the ovaxy, the pollen-tubes, when not immediately 

 led to the mouths of the ovules by special arrangements of the 

 conducting tissue, creep in a mostly very serpentine course along 

 the placenta, among the ovules, and finally penetrate singly, or 

 several together^ into the micropyle canals of the ovules. 



Ohserv. A considerable time elapsed from Amici's first observation on 

 the emission of the pollen-tubes upon the stigma of Fortulaca (1823), 

 before their further path to the ovule was detected ; for though Brong- 

 niart (1826) demonstrated, by numerous obfeervations, that the pollen-tubes 

 penetrated the conducting tissue, he thought he found that theb lower 

 endb burst, and that their fovilla was conveyed to the ovules by the con- 

 ducting tissue. Amici (1830, ^^Ann. cl Sc. nat'' xxi, 329) discovered the 

 perfect course to the ovule, but even in 1832, Eobert Brown was still in 

 doubt whether the tubes penetrating the ovules of the Orchidese were 

 pollen- tubes, or, more probably, tubes termed in the style, and to which 

 he applied the name of mibcous tubes, a doubt which was com|>letely 

 settled by Amici's researches, as was also the opinion advanced by many 

 later observers, that this phenomenon does not occur in all the Phanero- 

 gamia : — shewn to be totally mistaken, by the extensive researches of 

 Sclileiden, Hofmeister, (fee. 



It is one of the most puzzling phenomena existing, that the ends of the 

 pollen-tubes reach the micropyles of the ovules, the admibsion to which is 

 not always very simple ; since this rencontre seems to be lefl to pure acci- 

 dent, it might be conjectm^ed that for this purpose a very large number 

 of pollen-tubes were necessary. Yet such is not the case. It is tx-ue 

 that in the majority of plants, the number of pollen-tubes which are deve- 

 loped upon the stigma is very considerable, and we not unfrequently see 

 whole bundles of them penetrate the ovary, which is readily accounted 

 for by the vast number of pollen-grains found in the flowei^s, a tolerable 

 proportion of "which generally reach the stigma. Thus Kdlreuter found 

 4863 pollen-grains in the flower Hibiscus Trionum^ and according to 

 Amici's esthnate the pollen-grains of an anther of Orchis Morio can fur- 

 nish 120,000 pollen-tubes. But the number of poUen-gi'-ains necessary 

 for impregnation is by no means large. For example, in Kolreuter's 

 experiments on Hibiscus Trionum, 50 — 60 pollen-grains sufficed to impreg- 

 nate all the ovules in the ovary (over 30) ; when fewer pollen-grains 

 were placed upon the stigma the ovules were not all impregnated, for in- 

 stance, by 25 pollen-grains only 10 — 16 ovules. In Mi/rahiMs Jah/pa and 

 hngiflora one, or at most three, sufficed to impregnate the ovule. 



It is not necessary to the success of an impregnation that the pollen 

 should pass immediately from the anther to the stigma, for it seems to 

 remain capable of fertilizing for some days in all plants, while in some it 

 retains its power even for a year. Thus Kdlreuter found that the pollen 

 of Hibiscus Tfiovhum kept fresh three days, that of Ckei/ramtlms Ghm/r% 

 fom^teen days ; the pollen of Fhmnix dactylifora is said to be capable of 



