Chap. XL IN RELATION TO CROSS-FERTILISATION. 425 



memory of former visits 3ould have come into play, 

 and the tinge of blue was so faint that it could hardly 

 have served as a guide.* 



The conspicuousness of the corolla does not suffice 

 to induce repeated visits from insects, unless nectar is 

 at the same time secreted, together perhaps with 

 some odour emitted. I watched for a fortnight many 

 times daily a wall covered with Linaria cymbalaria 

 in full flower, and never saw a bee even looking at 

 one. There was then a very hot day, and suddenly 

 many bees were industriously at work on the flowers. 

 It appears that a certain degree of heat is necessary for 

 the secretion of nectar; for I observed with Lobelia 

 erinus that if the sun ceased to shine for only half an 

 hour, the visits of the bees slackened and soon ceased. 

 An analogous fact with respect to the sweet excretion 

 from the stipules of Vicia sativa has been already 

 given. As in the case of the Linaria, so with Pedicu- 

 laris sylvatica, Polygala vulgaris, Viola tricolor, and some 

 species of Trifolium, I have watched the flowers day 

 after day without seeing a bee at work, and then sud- 

 denly all the flowers were visited by many bees. Now 

 how did so many bees discover at once that the flowers 

 were secreting nectar ? I presume that it must have been 

 by their odour ; and that as soon as a few bees began 

 to suck the flowers, others of the same and of different 

 kinds observed the fact and profited by it. We shall 

 presently see, when we treat of the perforation of the 

 corolla, that bees are fully capable of profiting by the 



* A fact mentioned by H. flowers of the long-styled form, in 



Miiller ('Die Befruchtung,' &c, which the anthers are seated low 



p. 347) shows that bees possess down in the tubular corolla. Yet 



acute powers of vision and dis- the difference in aspect between 



crimination ; for those engaged in the long-styled and short-styled 



collecting pollen from Primula forms is extremely slight. 

 elatior invaririt \y passed by the 



