212 SCIENCE PROGRESS. 



reproductive purposes), the various flowers upon an inflores- 

 cence, the different parts of the same flower, the fruit, etc. 

 The amount of material available to the plant is not unlimited, 

 but depends upon the light-energy available and the amount 

 and efficiency of the chlorophyll-apparatus. If a large ex- 

 penditure be made in one direction, the plant must retrench 

 in another. A beginning has been made in this direction 

 by Vochting's work, already mentioned, and by that of 

 Burkill and Tansley upon the variation in number of the 

 parts of the flower. In flowers such as Ranunculaces and 

 Rosacese with indefinite stamens and carpels, the number 

 of these decreases on the whole in passing up a raceme or 

 down a cyme, and from the beginning to the end of the 

 flowering season. 



Of the remaining phenomena which come under this 

 heading, little need be said. We are no nearer, apparently, 

 to an understanding of the riddle of heterostylism than 

 when Darwin's book appeared. Burck has shown that the 

 Cassias, whose flowers have the style bent either to right 

 or left, are really self- fertilising, so that Midler's idea that 

 we have here a parallel case to ordinary heterostylism, must 

 be given up. M tiller is also responsible for another fanci- 

 ful "adaptation" -the "sun" and "shade" forms of Euphrasia, 

 Lysimackia, etc. He endeavoured to prove that the plants 

 growing in shady places had smaller flowers, better adapted 

 to self-fertilisation, than those in sunny spots. Whilst on 

 the whole this has some truth in it, it cannot be called an 

 adaptation, for we find both forms almost equally common in 

 either situation, along with every possible intermediate con- 

 dition. It seems in fact to be more a case of individual 

 variability than anything else. 



THE COLOURS OF FLOWERS. 



These are usually regarded as being adaptations to insect 

 visitors. Amongst the highest groups of insects, such as 

 the butterflies, bees, drone-flies (Syrphidse), etc., there is 

 no doubt a considerable sense of colour difference, as 

 Miiller and others have pointed out. But among the lower 

 groups it is rather doubtful if such is the case. The more 



