12 PLANTING. 



with numerous minute pores by which moisture and air are emitted and 

 absorbed. The upper surface of the leaves of trees is supposed to be 

 destitute of pores: this part always turns its surface to solar light*. 



The leaves of forest-trees are either simple or compound; simple as in 

 the common beech, and compound as in the ash, where several smaller 

 loaves are attached to one foot-stalk. The foot-stalk of the leaf is ter- 

 minated by a -land, which in deciduous trees, or those that shed their 

 leave.- in autumn, becomes indurated, and at that season readily separates 

 from the branch or twig. 



The midrib of the leaf is merely a continuation of the. footstalk ; this 

 divides the body of the leaf longitudinally ; it may be compared to the stem 

 of the tree, for from it issue branches of various sizes, dispersed through 

 the substance of the leaf in order resembling those of the tree. The first 

 or largest series of fibres issue from the sides of the midrib, either in 

 an opposite direction, alternately, or irregularly, according to the species of 

 tree; from these secondary branches proceed a third and a fourth series, 

 not however in such straight lines as in the former, but curving and 

 anastoinising, or opening into each other in all directions, and, in this last 

 particular, they have a resemblance to the disposition of the minute blood- 

 vessels of the animal economy. The difference in the disposition of the 

 first and secondary fibres of the leaves is so constant in the individual 

 plants of different natural genera, that it affords a very clear discriminating 

 character by which they maybe distinguished from each other, in the same 

 manner as the wood of different kinds are identified by the concentric 

 circles and medullary rays before mentioned. 



The leaves of the oak (Quercus) have the secondary fibres few in num- 

 ber, and curved towards the sinuosities of the leaf ; the thiid series of fibres 

 are very prominently marked, and the fourth series extremely minute. The 

 haves of the Spanish chestnut (Castanea), belonging to the same natural 

 order but to a different genus, have the secondary fibres nearly straight, 

 the third series very numerous and curved alternately, the fourth series 

 nearly as large as the third, and if we examine and compare the wood of 

 the oak and the chestnut, we shall find equally marked distinctions between 

 them. 



In the beech (Fagus), which likewise belongs to the same natural order, 

 but to a different genus to the above, the secondary fibres of the leaves are 

 \<i\ pivrninent, and the third and fourth series minute, and of nearly an 

 equal si/e, and the texture of the wood is equally distinct from that of the 

 oak and the chestnut. 



That the lea\es of plants during the day emit oxygen gas or vital air, 

 and absorb carbonic acid gas or impure air, has long since been proved. 

 In the night or during darkness, vital air is absorbed by plants and un- 

 healthy air emitted, and it would appear by various experiments on this 



Important point, thai when the supply of carbonic acid gas from the air 



and soil is ureater, tin- emission of oxygen gas by the leaves during their 



usishine is also gnater, hence another cause of healthy plan- 



us improving the climate of their neighbourhood besides that of shelter. 



being the great organs for elaborating the sap and 



fitting it to become converted into all and every product of the tree, whether 

 timber, bark, SCM!S or fruit, render the facts relating to their structure and 

 functions of high interest to the planter and forester; for they point out the 

 danger of lessening their number beyond a certain extent, as in excessive 



* Vug a minute and interesting account of the varied forms of these pores, and of their 

 number on different plauts, see Tart J. ' of Vegetable Physiology,' published in the Library 

 of Useful Knowledge. 



