37o NATURAL SCIENCE. June. 



young leaves transpire more rapidly when outspread than when hang- 

 ing. As a brightly coloured cell-sap is often characteristic of young 

 foliage in the tropics, it has been suggested that some protection 

 might thereby be afforded against too strong thermal or chemical 

 action of the sun's rays ; and thermometric observations showed that 

 the surface layers of the red leaf reflected more heat than those of 

 the green, and conversely, that the green leaf absorbed more heat than 

 the red. That is to say, the red colouring-matter acts as a screen by 

 which the heating effect of the sun's rays is moderated. Another 

 observer has recently shewn that it also is a powerful agent in pro- 

 tecting the leaf from too intense light. 



We might suggest the common ash as a subject of investigation 

 on the same lines. The opening leaves are tinged with a beautiful 

 red from the sap contained in the hairs and outer cell-layers of the 

 young leaves and leaf-stalks. Plunging in hot water at once extracts 

 the dye, giving a brilliant crimson solution, with an acid reaction, be- 

 coming greenish when rendered alkaline with potash or soda. Not 

 many years ago a tree grew ready to hand outside the window of the 

 Botanical Laboratory at Cambridge. 



Some criticism is passed on Stahl's view of the functions of the 

 long tips of the leaves as means for running off water from the upper 

 surface. It is pointed out that in trees of Amhevstia nobilis, growing in 

 the Peradeniya Botanical Gardens, the rather long tips consist of 

 tissue that does not become so tough or leathery as the rest of the 

 eaf, and while uninjured in the pendent position, these thin extremities 

 are rapidly killed when the leaves rise up and expose themselves to 

 the sun. Moreover, the tips, which were very long in the earlier 

 stages, become, as the full green adult condition is assumed, less well 

 marked. If the pointed end is a "drip-tip," why then is it better 

 developed when it is useless ? — for by the young leaf's position no 

 rain falls directly on it, and water-drops splashed on it are at 

 once rolled off by the hairs which are then present but which disappear 

 with maturity. 



In addition, however, to these temporary advantages, the author 

 suggests an additional "permanent" advantage. Besides affording 

 protection to the young foliage, the pendent habit enables the branch 

 and petioles to wait in a safe position till the conditions of shade or 

 sun, as far as these are governed by the arrangement of other 

 branches, can, so to speak, be ascertained. Then on rising up, the 

 branch with its leaves meets conditions more akin to those under 

 which it must pass the rest of its existence than if it had grown out 

 horizontally from the first. By means of the waiting and the rising 

 up of the branch and the petiole, the shoot behaves like one of the 

 leaflets — both are able to adjust themselves to a nicety to the pre- 

 vailing conditions of sunlight. The two movements, those of the shoot 

 with its leaf and of the leaflets, are compared to a coarse and fine 

 adjustment. 



