482 Transactions of the Society. 



Elliptic Pollen. 



By far the greater number of flowers have elliptic pollen, as 

 was long ago pointed out by Fritsche* and Mohl.f KernerJ says 

 this applies to quite half of all flowering plants. Why is this form 

 so predominant ? 



My first idea was that it might be accounted for by some 

 difference in the mode in which the pollen was attached to the 

 insects by which it was borne from one flower to another. 



I examined a number of insects captured on flowers — Lepi- 

 doptera, Diptera, Coleoptera, and especially Hymenoptera — but the 

 pollen seemed attached, so to say, almost at random, and I was 

 obliged therefore, to give up this line of enquiry. 



I then turned my attention to the structure of the stigma. In 

 many plants it is papillose, the papillse being in some arranged in 

 lobules, as in Cucumis ; in others smooth, or nearly so. Figs. 5 

 and 6 give an illustration of a papillose stigma, that of Salix. 

 Figs. 3 and 4 represent a smooth stigma, that of Centaurea cyanus, 

 the Blue Cornflower. 



Indeed I doubt whether the form has in itself any advantage. 

 It must be remembered that the pollen is originally more or less 

 spherical, and only assumes its definite form as it dries and conse- 

 quently contracts. 



Fairbairn, the great engineer, in his valuable book, " Useful 

 Information for Engineers," has a chapter on the resistance of tubes 

 to pressure. He is not concerned with the mode in which they 

 give way, his point being the resistance which tubes of different 

 materials are able to ofter. He shows, however, that when they 

 do give way they collapse from three equidistant points, thus 

 assuming a tlu-ee-cornered or three-lobed section. 



Fig. 7 gives a fresh pollen grain of Ranunculus ficaria taken 

 from the anther, and figs. 8 and 9 the same after 20 minutes expo- 

 sure to dry air ; fig. 9 being an end view. 



Finally tlien I came to the conclusion that the common ellip- 

 soidal form has no reference to anj^ structure of the insect by which 

 it is carried, or of the pistil on which it is to be deposited, but is 

 due to the structure of the pollen itself It may be remarked that 

 in Centaurea cyanus and some other species of the genus which 

 differ from most Composites in having smooth pollen, the form also 

 is ellipsoidal. 



It would seem that as pollen is now constituted, the three lines 

 along which it contracts correspond to three lines along which the 

 extine is thinner than elsewhere, and which, being therefore weaker, 

 ofler less resistance to contraction. Fig. 10 is taken from a section 



* Bei. z. K. d. PoUen, Berlin. 1832. 



t Uber der Bau und die Formen der Pollenkorner. 1834. 



t Nat. Hist, of Plants, ii. 



