34 HETEROMORPHOUS FLOWERS. [chap. 



while the other half have, on the contrary, the 

 stamens at the top of the tube, and the pistil half- 

 way down (as shown in Fig. 39). Corresponding 

 differences occur in Polyanthus and Auricula, and 

 have long been known to gardeners, and even to 

 schoolchildren, by whom the two forms are distin- 

 guished as " thrum -eyed " and " pin-eyed." As 

 already mentioned, plants which present these differ- 

 ences are known as Heteromorpkous (in opposition to 

 those which are Homomorphous, or have only one 

 kind of flower), while those with two forms are called 

 Dimorphous, those with three, Trimorphous. 



Sprengel himself had noticed a case of Dimorphism 

 in Hottonia, and shrewdly observed that there was 

 probably some reason for it, but was unable to 

 suggest any explanation. 



In Lythrum the existence of different forms had 

 been observed by Vaucherin 1841, and in the genus 

 Oxalis by Jacquin, who regarded them as indicative of 

 different species ; but it was reserved for the genius 

 and perseverance of Mr. Darwin to explain (Jour. 

 Linn. Soc. 1862, p. 'jf) the significance of this curious 

 phenomenon, and the important part it plays in the 

 economy of the flower. Now that Mr. Darwin has 

 pointed this out, it is sufficiently obvious : An insect 

 thrusting its proboscis down a primrose of the long- 

 styled form (Fig. 39) would dust its proboscis at a 

 part which, when it visited a short-styled flower 

 (Fig. 40), would come just opposite the head of the 

 pistil, and could not fail to deposit some of the 

 pollen on the stigma. Conversely, an insect visiting 

 a short-styled plant, would dust its proboscis at a 



