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THE POPULAR SCIENCE MONTHLY. 



approximately to the same level, and carrying the lower and outer 

 ones free from the upper and younger ones. 



Now, if we arrange the Spanish-chestnut leaves round a center, as 

 in Fig. 12, it is at once obvious how much space is wasted. On the 

 other hand, if we attach the leaves of the Acer to the stalk of Castanea 

 at the points from which the leaves of Castanea came off, as in Fig. 13, 



Fig. 12. 



Fig. 13. 



we shall see that the stalks are useless, and even mischievous, as a cause 

 of weakness and of waste of space ; while, on the other hand, if we 

 omit the stalks, or shorten them to the same length as those of Cas- 

 tanea, as in Fig. 14, the leaves would greatly overlap one another. 



Once more, for leaves arranged as in the beech the gentle swell at 

 the base is admirably suited ; but in a crown of leaves, such as those of 



the sycamore, space would be wasted, and it 

 is better that they should expand at once as 

 soon as their stalks have borne them free 

 from those within. Moreover, the spreading 

 lobes leave a triangular space (Fig. 11) with 

 the insertion of the stalk at the apex, which 

 seems as if expressly designed to leave room 

 for the pointed end of the leaf within. 



Hence we see how beautifully the whole 

 form of these leaves is adapted to the mode 

 of growth of the trees themselves and the 

 arrangement of their buds. 

 Before we proceed to consider the next series of species to which I 

 wish to direct attention, it will be necessary for me to say a few words 

 on the microscopical structure of the leaf. Although so thin, the leaf 

 consists of several layers of cells. Speaking roughly, and as a general 

 rule, we may say that on each side is a thin membrane, or epidermis, 

 underneath which on the upper side are one or more layers of elongated 



Fig. 14. 



