TREES AND SHRUBS FOR LARGE TOWNS. 



83 



have to contend with a smaller dose of smoke than those which 

 unfold their leaves earlier in the spring. Trees, again, which 

 retain their foliage late are preferable to those in which the 

 leaves fall early. The comparison in this respect between 

 the Plane and the Lime is wholly in favour of the Plane. 

 Trees with thin, flaccid leaves, like those of the Lime, are 

 obviously at a disadvantage as compared with others in which 

 the leaves are firmer and more resistent. The former shrivel 

 with heat and drought much sooner than the latter. Trees 

 like the Lime, again, from the leaves of which exudes a gummy 

 or saccharine fluid, are unsuitable, inasmuch as the dust 

 adheres to them and is not readily dislodged by rain, while 

 the sweet juices are as nectar to undesirable insects, On the 

 other hand, leaves of firm texture with glossy skins, which are 

 soon cleansed, such as those of the Tulip-tree, the Cucumber- 

 tree (Magnolia acuminata), the Naples Alder (Alnus cordata), 

 some of the Poplars, many Maples, and the Copper Beech, are 

 well suited for towns. The Maidenhair-tree (Ginkgo biloba) is ex- 

 cellently adapted for planting in towns, though rarely seen. 

 In Brentford, in close vicinity to the gasworks of that noisome 

 town, in the Botanic Garden at Chelsea, and in the Bow Boad, 

 fine trees of this species — isolated examples of course — may still 

 be seen. A superficial examination of the leaves of this species, 

 and of their mode of attachment, direction, and disposition, will 

 show how well adapted they are to resist the injurious effects of 

 a polluted atmosphere. Microscopical examination of these 

 leaves (as also those of the Iris, Carnation, and Auricula among 

 herbaceous plants, and which are all good town plants) will con- 

 firm the results of superficial examination by revealing a tough 

 and relatively thick epidermis, abundance of breathing pores, and 

 a relatively large amount of leaf-green or " chlorophyll." An 

 adequate supply of this latter substance is to the plant what a 

 corresponding supply of healthy blood is to the animal. Plants 

 richly endowed with it have naturally greater powers of resist- 

 ance than those less well provided. The thing is obvious to 

 truism. 



Evergreens are called on to exert an even greater power of 

 resistance than are deciduous shrubs, but their glossy leaf- 

 surface, the thick texture of their leaves, and their accumulations 

 of deep green chlorophyll enable them to maintain themselves 



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