WICIITTRA ON HYBRIDS. 



05 



than amongst hybrid willows. But what most concerns us afc 

 present is the remark of Graertner, that in the anthers of all fertile 

 hybrids, as for example in Nicotiana rustica paniculata, Malva 

 onauritiana sylvestris, Aquilegia atropurpurea canadensis, Lychnis 

 diurna vespertina, larger and smaller grains are mixed in different 

 proportions, besides small grains of different degrees of irregu- 

 larity, as elongated, shrivelled, or mere arid sacs ; and this is the 

 case especially with those which are the least fertile, a fact which 

 was observed also by Koelreuter. A tolerably correct opinion, 

 therefore, may be formed of the comparative fertility of hybrids 

 from an examination of the pollen, as appears also very distinctly 

 from Wichura's observations. 



Pollen-grains of willows were treated with the above-mentioned 

 solution of honey ; and it appeared, on microscopical examination, 

 that the normally formed grains of hybrids, of a light colour and 

 semitransparent aspect, constantly developed pollen-tubes ; those 

 like No. 5 usually did so ; the irregular grains like No. 2, if of a 

 light colour, occasionally ; while the confluent grains, of a dark, 

 dirty yellow hue, with those comprised nnder Nos. 1, 3, 4, and 6, 

 never made pollen-tubes, and must be regarded as absolutely sterile. 



It appeared also, on comparing the activity of the pollen-grains 

 of hybrids with those of pure species, that there was a difference. 

 The defect in hybrid pollen in some hybrids, and probably in a 

 great many, though not in all, was twofold. The number of tube- 

 producing grains is far smaller than in pure species, while the 

 potency of these grains is comparatively weak. 



The study of the pollen-grains in willows is comparatively easy ? 

 as the contents of the anthers in every stage are easily squeezed 

 out on the stage of the microscope by the covering glass, so that 

 they are ready for examination without any further preparation. 

 Willow-branches, moreover, are readily developed in water, and 

 the different stages of development of the pollen observed without 

 much trouble. 



In pure willows the mother cells of the anthers are of equal 

 size, and divide, with almost mathematical precision, into four 

 tetrahedric pockets, in which the contents form yellowish shining 

 pollen-grains of exactly the same size ; while in hybrids the course 

 of development is far less regular, and subject to the above-men- 

 tioned derangements. 



The first form arises from the fact that the component cells of 

 the substance of the anthers (Collenchymzellen) , contrary to the 

 established rule, do not dissolve, but still hang together, and thus 



VOL. I. p 



