LESSON ON LEAVES. 



279 



of the leaf, and are composed of woody 

 fibre with a central pith similar to the woody 

 parts of the stem and branches. They are 

 in fact the farthest extensions of the branch- 

 ing- of the tree, and convey the sap to the 

 remotest parts of the leaf where it can be 

 spread out and exposed to sunlig-ht. 



The spaces between the veins are made 

 un of a soft, cellular substance, filled with 

 minute chlorophyll granules which give to 

 the leaf its green coloring matter. The 

 whole structure is covered above and below 

 with a thin transparent skin or epidermis, 

 through which may be seen, when the leaf 

 is placed under a microscope, numerous 

 small openings called the stomata. Each 

 stoma is an automatic valve by means of 

 which the leaf performs its most important 

 functions. 



THE FUNCTIONS OF LEAVES 



A careful study of all of the functions per- 

 formed by the leaves and the chemical 

 changes which take place in them would 

 lead us farther afield than we have time at 

 present to go. Those which are more 

 directly dependent upon the care and mana- 

 gement of the tree, it is important, however 

 for us to consider. 



THE TRANSPIRATION OF MOISTURE 



The soil water which is taken up by the 

 roots is to a great extent merely a carrying 

 agent, and the greater portion of has to be 

 gotten rid of after it has brought the plant 

 food from the soil to the leaves. This 

 giving off of the water takes place through 

 the stomata. It is this transpiration of 

 water from the leaves which causes plants 

 to wilt, when in a very hot sun or dry at- 

 mosphere, the direct cause of the wilting 

 being that the leaves are giving off the 

 moisture faster than the roots can take it 

 up. 



The stomata open and close according to 

 the conditions surrounding them. In hot, 



dry weather, particularly when it is windy, 



they are inclined to open and give off water 



very rapidly. Hence it is important during 



such seasons to have the soil, in which the 



plant is growing, well cultivated so that it 



may retain plenty of moisture where the 



roots can get at it. 



As the roots absorb water and the leaves 



give it off, there must be an equilibrium 



between the roots and the leaves of the 



plant, if it is to maintain growth. Hence 



when trees are taken up and transplanted 



and the greater part of the root-system is 



destroyed, it is usually necessary to cut 



back the top to correspond with the roots 



left. 



THE LEAVES ACT AS LUNGS 



The leaves are often spoken of as the 

 plant. They inhale carbonic acid gas and 

 exhale oxygen, just the reverse of what 

 takes place in the breathing of animals. In 

 this respect, animal and plant life are com- 

 plimentary one to the other. As it is im- 

 portant for the health of animals that they 

 have vigorous, strong lungs, so it is im- 

 portant for the growth of the plant that it 

 has vigorous, healthy foliage. For this 

 reason, it often becomes necessary to spray 

 the foliage of trees, even when not bearing, 

 where they are subject to the attacks of 

 fungus diseases which develop in the foliage; 

 for without healthy foliage, the tree will 

 not make satisfactory growth. 



Another important function of the leaves 

 is to assimilate or make use of the plant 

 food taken in from the soil and air. In this 

 respect the leaves might also be called the 

 digestive organs of the plant. The car- 

 bonic acid gas taken in by the leaves is 

 combined with the sugar of the sap forming 

 starch. This formation of starch is brough 

 about in the chlorophyll granules by the 

 action of sunlight. If the leaves do not get 

 sufficient light, or if the foliage has been 

 half eaten by insects, the plant will be 

 starved to that extent. 



