1885.] on the Forms of Leaves. 199 



life produces both leaves and phyllodes, which give it a very curious 

 and interesting appearance. In Eucalyptus, again, the young plant 

 has horizontal leaves, which in older ones are replaced by scimitar- 

 shaped phyllodes. Hence the different appearance of the young and 

 old trees which must have struck every visitor to Algiers or the 

 Riviera. 



Evergreens. 



We have hitherto been considering mainly deciduous trees. In 

 evergreens the conditions are in many respects different. It is 

 generally said that leaves drop off in the autumn because they die. 

 This, however, is not strictly correct. The fall of the leaf is a vital 

 process, connected with a change in the cellular tissue at the base of 

 the leaf-stalk. If the leaves are killed too soon they do not drop off. 

 Sir John illustrated this by some twigs which he had pui'posely 

 broken in the summer ; below the fracture the leaves had been thrown 

 off, above they still adhered, and so tightly that they could support a 

 considerable weight. In evergreen trees the conditions are in many 

 respects very different. It is generally supposed that the leaves last 

 one complete year. Many of them, however, attain a much greater 

 age ; for instance, in the Scotch Fir, two or three years ; in the 

 Spruce and Silver, six or seven ; in the Yew even longer. It appears 

 from this that they require a tougher and more leathery texture. 

 When we have an early fall of snow our deciduous trees are often 

 much broken down ; glossy trees have a tendency to throw it off, and 

 thus esca]3e ; hence evergreen leaves are very generally smooth and 

 glossy. Again, evergreen leaves often have special protection, either 

 in an astringent or aromatic taste, which renders them more or less 

 inedible ; or by thorns and spines. Of this the Holly is a familiar 

 illustration ; and it was j)oiiited out that in old plants, above the 

 range of browsing quadrupeds, the leaves tend to lose their spines, 

 and become unarmed. The hairs on leaves are another form of pro- 

 tection ; on herbs, the j)resence of hairs is often associated with that 

 of honey, as they protect the plants from the visits of creeping 

 insects ; hence perhajDS the tendency of water species to become 

 glabrous, Polygonum amjphihium being a very interesting case, since 

 it is hairy when growing on land, and smooth when in water. Sir 

 John then dealt with cases in which one sjpecies mimics another, and 

 exhibited a striking photograph of a group of Stinging Nettles and 

 Dead Nettles, which were so much alike as to be hardly distinguish- 

 able. No one can doubt that the Stinging Nettle is protected by its 

 poisonous hairs, and it is equally clear that the innocuous Dead 

 Nettle must profit by its similarity to its dangerous neighbour. Other 

 similar cases were cited. 



He had already suggested one consideration which in certain 

 cases determined the width of leaves, but there were others in which 

 it was due to other causes, one being the attitude of the leaf itself. 



