334 FORMS OF LEA VES. [March 



In the yew, the leaves are inserted close to one another, and 

 are long and linear ; while in the box they are farther apart and 

 broader. In the Scotch fir, the leaves are linear, and 1^ inch long; 

 while in other pines, as for instance the Weymouth, the stem is 

 thicker and the leaves longer. 



In the plants hitherto mentioned, one main consideration appears 

 to be the securing of as much light as possible; but in tropical 

 countries the sun is often too powerful, and the leaves far from 

 courting, avoid the light. The typical acacias have pinnate leaves, but 

 in most Australian species the true leaves are replaced by a vertically 

 flattened leaf-stalk. It will be found, however, that the seedlings 

 have leaves of the form typical in the geuus. Gradually, however, 

 the leaf becomes smaller and smaller, until nothing is left but the 

 flattened leaf-stalk or phyllode. In one species the plant through- 

 oiit 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 appearances of 

 the young and old trees, which must have struck every visitor to 

 Algiers or the Eiviera. 



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 

 purposely 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 con- 

 ditions 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, three or 

 four years ; in the spruce and silver, six or seven ; in the yew, even 

 longer. It follows 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 leaves have a 

 tendency to throw it off and thus escape ; hence evergreen leaves 

 are very generally smooth and glossy. Again, evergreen leaves often 

 have special protection, either in an astringent or aromatic taste, 

 which render them more or less inedible, or by thorns and spines. 

 Of this the holly is a familiar illustration, and it was pointed 

 out that in old plants, above the range of browsing quadrupeds, the 

 leaves tend to lose their spines and become unarmed. The hair on 



