two most precious of all the elements for the growth of plants—­but in the case of most open flowers, a large quantity of pollen is consumed by pollen-devouring insects, and a large quantity is destroyed during long-continued rain.  With many plants this latter evil is guarded against, as far as is possible, by the anthers opening only during dry weather (10/7.  Mr. Blackley observed that the ripe anthers of rye did not dehisce whilst kept under a bell-glass in a damp atmosphere, whilst other anthers exposed to the same temperature in the open air dehisced freely.  He also found much more pollen adhering to the sticky slides, which were attached to kites and sent high up in the atmosphere, during the first fine and dry days after wet weather, than at other times:  ‘Experimental Researches on Hay Fever’ 1873 page 127.)—­by the position and form of some or all of the petals,—­by the presence of hairs, etc., and as Kerner has shown in his interesting essay, by the movements of the petals or of the whole flower during cold and wet weather. (10/8.  ‘Die Schutzmittel des Pollens’ 1873.) In order to compensate the loss of pollen in so many ways, the anthers produce a far larger amount than is necessary for the fertilisation of the same flower.  I know this from my own experiments on Ipomoea, given in the Introduction; and it is still more plainly shown by the astonishingly small quantity produced by cleistogene flowers, which lose none of their pollen, in comparison with that produced by the open flowers borne by the same plants; and yet this small quantity suffices for the fertilisation of all their numerous seeds.  Mr. Hassall took pains in estimating the number of pollen-grains produced by a flower of the Dandelion (Leontodon), and found the number to be 243,600, and in a Paeony 3,654,000 grains. (10/9.  ‘Annals and Magazine of Natural History’ volume 8 1842 page 108.) The editor of the ‘Botanical Register’ counted the ovules in the flowers of Wistaria sinensis, and carefully estimated the number of pollen-grains, and he found that for each ovule there were 7000 grains. (10/10.  Quoted in ‘Gardeners’ Chronicle’ 1846 page 771.) With Mirabilis, three or four of the very large pollen-grains are sufficient to fertilise an ovule; but I do not know how many grains a flower produces.  With Hibiscus, Kolreuter found that sixty grains were necessary to fertilise all the ovules of a flower, and he calculated that 4863 grains were produced by a single flower, or eighty-one times too many.  With Geum urbanum, however, according to Gartner, the pollen is only ten times too much. (10/11.  Kolreuter ‘Vorlaufige Nachricht’ 1761 page 9.  Gartner ‘Beitrage zur Kenntniss’ etc. page 346.) As we thus see that the open state of all ordinary flowers, and the consequent loss of much pollen, necessitate the development of so prodigious an excess of this precious substance, why, it may be asked, are flowers always left open?  As many plants exist throughout the vegetable kingdom which bear cleistogene flowers, there can hardly be a doubt that all open flowers might easily have been converted into closed ones.  The graduated steps by which this process could have been effected may be seen at the present time in Lathyrus nissolia, Biophytum sensitivum, and several other plants.  The answer to the above question obviously is, that with permanently closed flowers there could be no cross-fertilisation.