Mr. Hassall on the Structure of the Pollen Granule.. 107 



If the joint of the filament be touched where it protrudes 

 between the petals, no such effect as I have described will 

 be produced, but the moment the hair is touched it takes 

 place j now the presence of these hairs affords a very inter- 

 esting instance of design. They are connected with the joint 

 of each petal by means of a raised line of elastic tissue which 

 runs along their centre, and which doubtless serves to convey 

 the impression or shock, imparted to the hairs most frequently 

 by means of insects, to the joint, causing it to contract ; the 

 dispersion of the pollen and consequent fecundation of the 

 ovule being thereby rendered more certain. 



The second instance to which I shall refer is witnessed in 

 the common stinging nettle, Urtica dioica : the number of 

 stamina in this plant is likewise four ; these are inclosed in a 

 chalice or cup composed of four sepals, and the filaments are 

 coiled inwards ; on the reflection of these sepals, or on the 

 occurrence of any shock, the filaments disengage themselves, 

 and not merely straighten, but turn as much outwards as they 

 were previously coiled inwards, the pollen being scattered, as 

 in the former case, around them. If the filament of each sta- 

 men be examined with the microscope, one surface of it, that 

 is, the one which formed the concavity before its disengage- 

 ment, but after, the convexity, is observed to be ringed, in the 

 same way as the elastic spring of the sporangia of Ferns. An 

 analogous instance of irritability occurs in Parietaria, an al- 

 lied genus*. 



The stigmata of some plants, as for example of Pavetta 

 Caffra, the Campanula &c, extend so much beyond and 

 above the anthers, that it is difficult to conceive in what way 

 the pollen can reach them ; but observation affords an expla- 

 nation of the means : at the time of the bursting of the anthers 

 the stigma is on a level with them, and, apparently stimulated 

 by contact with the pollen grains, subsequently rises up, carry- 

 ing with it in its progress a quantity of the pollen. 



Moreover, the application of the pollen to the stigma is ren- 

 dered more certain by a cause, which, when not rightly consi- 

 dered, may appear trifling, but which, in its results, is far from 

 being so. I allude to the agency of insects, and of these, 

 especially to the Aphides, and our benefactor the Bee, that 

 busy labourer for man, who in rifling a flower of part of its 

 sweets, yet in doing so confers upon it the means of its per- 

 petuity by dispersing the pollen around, some of which never 

 fails to reach the stigma. To many insects the pollen doubt- 



* I now know that this second example of vegetable irritability or elasti- 

 city of tissue is alluded to in Lindley's ! Natural Arrangement,' and also 

 in Henslow's ' Descriptive and Physiological Botany.' 



