eal scientific interest。 Kolreuter mentioned that pollen may be carried from one flower to another partly by wind and partly by insects。 But he held the view； and that was； indeed； the natural assumption； that self… fertilisation usually occurs in a flower； in other words that the pollen of a flower reaches the stigma of the same flower。 He demonstrated； however； certain cases in which cross…pollination occurs； that is in which the pollen of another flower of the same species is conveyed to the stigma。 He was familiar with the phenomenon； exhibited by numerous flowers； to which Sprengel afterwards applied the term Dichogamy； expressing the fact that the anthers and stigmas of a flower often ripen at different times； a peculiarity which is now recognised as one of the commonest means of ensuring cross…pollination。
With far greater thoroughness and with astonishing power of observation C。K。 Sprengel （1750…1816） investigated the conditions of pollination of flowers。 Darwin was introduced by that eminent botanist Robert Brown to Sprengel's then but little appreciated work；〃Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen〃 （Berlin； 1793）； this is by no means the least service to Botany rendered by Robert Brown。
Sprengel proceeded from a naive teleological point of view。 He firmly believed 〃that the wise Author of nature had not created a single hair without a definite purpose。〃 He succeeded in demonstrating a number of beautiful adaptations in flowers for ensuring pollination； but his work exercised but little influence on his contemporaries and indeed for a long time after his death。 It was through Darwin that Sprengel's work first achieved a well deserved though belated fame。 Even such botanists as concerned themselves with researches into the biology of flowers appear to have formerly attached much less value to Sprengel's work than it has received since Darwin's time。 In illustration of this we may quote C。F。 Gartner whose name is rightly held in the highest esteem as that of one of the most eminent hybridologists。 In his work 〃Versuche und Beobachtungen uder die Befruchtungsorgane der vollkommeneren Gewachse und uber die naturliche und kunstliche Befruchtung durch den eigenen Pollen〃 he also deals with flower…pollination。 He recognised the action of the wind； but he believed； in spite of the fact that he both knew and quoted Kolreuter and Sprengel； that while insects assist pollination； they do so only occasionally； and he held that insects are responsible for the conveyance of pollen； thorough investigations would show 〃that a very small proportion of the plants included in this category require this assistance in their native habitat。〃 （Gartner； 〃Versucher und Beobachtungen。。。〃； page 335； Stuttgart； 1844。） In the majority of plants self…pollination occurs。
Seeing that even investigators who had worked for several decades at fertilisation…phenomena had not advanced the biology of flowers beyond the initial stage； we cannot be surprised that other botanists followed to even a less extent the lines laid down by Kolreuter and Sprengel。 This was in part the result of Sprengel's supernatural teleology and in part due to the fact that his book appeared at a time when other lines of inquiry exerted a dominating influence。
At the hands of Linnaeus systematic botany reached a vigorous development； and at the beginning of the nineteenth century the anatomy and physiology of plants grew from small beginnings to a flourishing branch of science。 Those who concerned themselves with flowers endeavoured to investigate their development and structure or the most minute phenomena connected with fertilisation and the formation of the embryo。 No room was left for the extension of the biology of flowers on the lines marked out by Kolreuter and Sprengel。 Darwin was the first to give new life and a deeper significance to this subject； chiefly because he took as his starting…point the above…mentioned problems； the importance of which is at once admitted by all naturalists。
The further development of floral biology by Darwin is in the first place closely connected with the book on the fertilisation of Orchids。 It is noteworthy that the title includes the sentence；〃and on the good effects of intercrossing。〃
The purpose of the book is clearly stated in the introduction：〃The object of the following work is to show that the contrivances by which Orchids are fertilised； are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom； and； secondly； to show that these contrivances have for their main object the fertilisation of each flower by the pollen of another flower。〃 （〃Fertilisation of Orchids〃； page 1。） Orchids constituted a particularly suitable family for such researches。 Their flowers exhibit a striking wealth of forms； the question； therefore； whether the great variety in floral structure bears any relation to fertilisation （In the older botanical literature the word fertilisation is usually employed in cases where POLLINATION is really in question： as Darwin used it in this sense it is so used here。） must in this case possess special interest。
Darwin succeeded in showing that in most of the orchids examined self… fertilisation is either an impossibility； or； under natural conditions； occurs only exceptionally。 On the other hand these plants present a series of extraordinarily beautiful and remarkable adaptations which ensure the transference of pollen by insects from one flower to another。 It is impossible to describe adequately in a few words the wealth of facts contained in the Orchid book。 A few examples may； however； be quoted in illustration of the delicacy of the observations and of the perspicuity employed in interpreting the facts。
The majority of orchids differ from other seed plants （with the exception of the Asclepiads） in having no dust…like pollen。 The pollen； or more correctly； the pollen…tetrads； remain fastened together as club…shaped pollinia usually borne on a slender pedicel。 At the base of the pedicel is a small viscid disc by which the pollinium is attached to the head or proboscis of one of the insects which visit the flower。 Darwin demonstrated that in Orchis and other flowers the pedicel of the pollinium； after its removal from the anther； undergoes a curving movement。 If the pollinium was originally vertical； after a time it assumed a horizontal position。 In the latter position； if the insect visited another flower； the pollinium would exactly hit the sticky stigmatic surface and thus effect fertilisation。 The relation between the behaviour of the viscid disc and the secretion of nectar by the flower is especially remarkable。 The flowers possess a spur which in some species （e。g。 Gymnadenia conopsea； Platanthera bifolia； etc。） contains honey （nectar）； which serves as an attractive bait for insects； but in others （e。g。 our native species of Orchis） the spur is empty。 Darwin held the opinion； confirmed by later investigations； that in the case of flowers without honey the insects must penetrate the wall of the nectarless spurs in order to obtain a nectar…like substance。 The glands behave differently in the nectar…bearing and in the nectarless flowers。 In the former they are so sticky that they at once adhere to the body of the insect； in the nectarless flowers firm adherence only occurs after the viscid disc has hardened。 It is； therefore； adaptively of value that the insects should be detained longer in the nectarless flowers （by having to bore into the spur）；than in flowers in which the nectar is freely exposed。 〃If this relation； on the one hand； between the viscid matter requiring some little time to set hard； and the nectar being so lodged that moths are delayed in getting it； and； on the other hand； between the viscid matter being at first as viscid as ever it will become； and the nectar lying all ready for rapid suction； be accidental； it is a fortunate accident for the plant。 If not accidental； and I cannot believe it to be accidental； what a singular case of adaptation！〃 （〃Fertilisation of Orchids〃 （1st edition）； page 53。）
Among exotic orchids Catasetum is particularly remarkable。 One and the same species bears different forms of flowers。 The species known as Catasetum tridentatum has pollinia with very large viscid discs； on touching one of the two filaments （antennae） which occur on the gynostemium of the flower the pollinia are shot out to a fairly long distance （as far as 1 metre） and in such manner that they alight on the back of the insect； where they are held。 The antennae have； moreover； acquired an importance； from the point of view of the physiology of stimulation； as stimulus… perceiving organs。 Darwin had shown that it is only a touch on the antennae that causes the explosion； while contact； blows； wounding； etc。 on other places produce no effect。 This form of flower proved to be the male。 The second form； formerly regarded as a distinct species and named Monachanthus viridis； is shown to be the female flower。 The anthers have only rudimentary pollinia and do not open； there are no antennae； but on the other hand numerous seeds are produced。 Another type of flower； known as Myanthu