POLLEN-GRAINS. 



429 



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

 water, and the fovUla escapes as a slightlj- coherent mass which 

 soon becomes more diffused and allows the finer granules to pass 

 into the water, where thej^ immediately exhibit the Brownian 

 movement, common to all minute particles suspended in a 

 liquid. "■ 



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

 stead of in pure water, they will increase somewhat in size ; 

 and in a few 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.^ 



1115. The pollen-grains of TulipaGesneriana 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.' 



1 For an extfniled 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 seq. ; and also the 

 reinarltable 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). 



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



Fig. 194. a, young pollen-gi'ain of AUium flstnlosum, before its division ; b, after tlie 

 division of tlie nucleus: c, after the division of the protoplasm; f?, young pollen-grain of 

 Monotropa Hypoi>itys divided; e, same eniitfiiig its tube, into which the two nuclei 

 pass; /. coalescent grains of the pollen of Platanthera bifolia during their division ; g^ 

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

 burger .J 



