n PRIMULACE^ 267 



Three are yellow-flowered : the Primrose, with peduncles 

 one-flowered and apparently radical; and the Cowslip and 

 Oxlip, in which the peduncles bear an umbel of several 

 flowers. The limb of the corolla in the Cowslip is small 

 and concave, in the Oxlip broad and flat. These three 

 were considered by Linnaeus and Bentham as varieties 

 of one species. Lastly, P. farinosa is pale lilac, and 

 is covered in parts with a white meal. All four are 

 dimorphous ; that is to say, if a number of specimens 

 are examined it will be found that about half of them 

 have the stigma at the top of the tube, and the stamens 

 half-way down (as is shown in Fig. 168); while the other 



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FlQ. 167. — Primula elatior. Section through the wall of a capsule, e, external epiderm ; 

 i, internal epiderm, partly lignified ; I, line of dehiscence ; p, parenchyma. 



half have, on the contrary, the stamens at the top of the 

 tube, and the stigma half-way down (as shown in Fig. 

 169). Corresponding difierences occur in Polyanthus and 

 Auricula. They have long been known to gardeners, and 

 even to school children, by whom the two forms are dis- 

 tinguished as " pin-eyed " and "thrum-eyed." As already 

 mentioned, plants which present these differences are 

 known as heteromorphous (in opposition to those which 

 are homomorphous, or have only one kind of flower). 

 Heteromorphous species 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 explana- 

 tion. In Lythrum the existence of different forms had 



