POLLEN-GRAiNS. 



429 



1113. The pollen-grains of mam- plants burst when placed in 

 water, and the fovilla escapes as a slightly coherent mass which 

 soon becomes more diffused and allows the finer granules to pass 

 into the water, where they immediately exhibit the Brownian 

 movement, common to all minute particles suspended in a 

 liquid. 1 



1114. If pollen-grains are placed in a solution of sugar in- 

 stead of in pure water, the}- will increase somewhat in size ; 

 and in a few 7 hours, if the specimen is kept at the right tem- 

 perature, there will appear at some point of the surface of each 



grain a minute tube, which by great care can be cultivated in a 

 proper medium until it attains a length of several millimeters.' 2 



1115. The pollen-grains of Tulipa Gesneriana emit their tubes 

 in a 1 to 3 per cent solution of cane-sugar ; the following require 

 a somewhat stronger syrup : Leucojum sestivum and Narcissus 

 poeticus, 3 to 5 per cent ; most orchids, 5 to 10 per cent ; Con- 

 vallaria majalis, 5 to 20 per cent ; Iris sibirica, 30 to 40 per 

 cent. 3 



1 For an extended account of the speculations once based upon the occur- 

 rence in water of motion of the particles of the fovilla, the reader should 

 consult Meyen: Pflanzenphysiologie, iii., 1839, pp. 192 ct scq. ; and also the 

 remarkable treatise by Robert Brown. 



2 Schleiden states that pollen-grains which come accidentally in contact 

 with nectar readily send out tubes ; and that we often find at the base of the 

 flower a whole mass of confervoid web, which consists of entangled pollen- 

 tubes emitted in this manner (Principles of Scientific Botany, 1849, p. 408). 



3 Strasburger : Das botanische Practicum, 1884, p. 511. 



FIG. 194. a, young pollen-grain of Allium fistulosum, before its division ; 6, after the 

 division of the nucleus; c, after the division of the protoplasm; d, young pollen-grain of 

 Monotropa Hypopitys divided ; e, same emitting its tube, into which the two nuclei 

 pass; /, coalescent grains of the pollen of Platanthera bifolia during their division ; g t 

 formation of the pollen-tube of Orchis mascula, into which the two nuclei pass. ( Stras- 

 burger J 



