Structure of Plants. Vj9 



Colour Protection from Heat. 



Another case which I could not at first explain was the dark 

 band seen on some garden geranium leaves. But when one sees 

 a young leaf developing, it is just the band that is exposed to 

 light. The edges turn up so as to shield the outer and inner 

 parts. This colour is due to a very interesting su?)stance called 

 amthocyan. It is summoned into existence by the injurious effect 

 of certain special rays' of light (ultra violet rays). But when formed 

 it protects the underlying tissue and especially guards certain 

 ferments or enzymes, such as diastase, from destruction by those 

 same ultraviolet rays. It is a light screen produced by light. 

 But in certain young Hieraciums or Hawkweeds one finds the 

 anthocyan on the under surface, not on the upper. Moreover, the 

 young leaves are vertical, densely hair}-, and require no 

 anthocyan. Is this inexplicable? These ferments, diastases, 

 and the like, will occur in the phloem of the leaf which is in the 

 lower spongy parenchyma. The upper part of the leaf or 

 pallisade parenchyma does not probably require anthocyan. 

 An alternative explanation has been offered, which is that these 

 hawkweeds grow on rocks and the strong heat radiated from the 

 heated rock surface would injure them. This may be the explana- 

 tion, but as a matter of fact these specimens did not grow on 

 rocks, but in short turf under beeches. 



The distribution of this anthocyan colour is very interesting 

 and peculiar so that I shall give two more cases. In the Colum- 

 bine there is a tiny edge or dot of anthocyan and a small hard 

 point at the tip of the leaflet. A young leaf shows that it is just 

 those parts that are exposed during development, for the leaflets 

 are all nearly vertical. Then again the leaves of the wood 

 anamone have the projecting veins of the underside reddened. 

 This seems useless, but then when developing the young anemone 

 leaf is bent over in a curious knee-like manner. Moreover, the 

 veins are closed together, and obvious the red colour is just where 

 it ought to occur. You will see, therefore, that it is not at all 

 easy to discover useless characters. I do not reallv know how 

 many flowers and leaves I have examined, but I must have 

 seen several thousands of species, and it is my firm belief that 

 unnecessary characters do not occur in natural species. The 

 point is of some theoretical importance. If there really is a 



