386 



HUTCHINSON'S POPULAR BOTANY 



Primroses, which they know respectively as pin-eyed and thrum-eyed. In 

 the pin-eyed form, only the little round stigma is visible at the top of 

 the pipe, while the stamens, here joined with the corolla-tube, hang like 

 little bags half-way down the neck of it. In the thrum-eyed form, on the 

 other hand, only the stamens are visible at the top of the tube, while 

 the stigma, erected on a much shorter style, occupies just the same place in 

 the tube that the stamens occupied in the sister blossom. Now each 



Primrose-plant bears only 

 one form of flower. There- 

 fore, if a bee begins visiting 

 a thrum-eyed form, he will 

 collect pollen on his probos- 

 cis at the very base only ; 

 and as long as he goes on 

 visiting thrum- eyed flowers, 

 he can only collect, without 

 getting rid of any grains on 

 the deep-set stigmas. But 

 when he flies away to a pin- 

 eyed blossom, the part of his 

 proboscis which collected 

 pollen before will now be 

 opposite the stigma, and will 

 fertilize it ; while at the same 

 time he will be gathering 

 fresh pollen below, to be 

 rubbed off on the sensitive 

 surface of a short-styled 

 flower in due season. Thus 

 every pin-eyed blossom must 

 always be fertilized by a 

 thrum-eyed, and every 

 thrum-eyed by a pin-eyed 

 neighbour." 



Heterostylism is carried 

 to an extreme in trimorphic 



flowers, where, besides a long- and a short-styled form, we get a third 

 condition, to which Darwin has applied the name " mid-styled." The flowers 

 of Purple Loosestrife (Lythrum saticaria, figs. 445, 465) offer, perhaps, the 

 best examples of trimorphism. The three forms are shown with diagram- 

 matic stiffness in fig. 466, the floral envelopes (calyx and corolla) having 

 previously been stripped off. Comparing the different forms : in the left- 

 hand figure there are six short stamens, six mid-length stamens, and a 

 long style ; in the central figure there are six short stamens, six long ones, 



FIG. 475. FLOWER-BUDS OP Aralia nudicaulis. 



ingle flow< 



(a) Umbel of unopened flowers ; (6) 

 (c) umbel at an ear 



3rmal growth of 

 stage of growth . 



