CHAPTER III 



The devices to meet the danger of excessive transpiration are the 

 most striking features of our dry-weather plants. By all means, 

 emphasize the fact of transpiration by letting the children perform 

 the experiments. Ordinary quart fruit jars will do, and if rather large 

 quantities of plants are used, the weights or scales need not be very 

 fine. Of course metric weights are best. If practicable, have the 

 children seal the stems with sealing wax or grafting wax. These 

 simple experiments do not give very definite or accurate results, but 

 the facts that leaves exhale varying amounts of water and that the 

 epidermis restricts evaporation are clearly shown. Of course children 

 can be taught that ordinarily the evaporation of water from plants is 

 invisible, but it is well to make the vapor visible by using the closed 

 jars. On a warm autumn day this requires less than ten minutes. 

 Evaporation soon ceases in the jars because the air becomes saturated, 

 so if the loss of water is to be determined by weight the plants must 

 be exposed to dry air. 



As previously stated, water escapes from leaves, not through the 

 epidermal cells, but through the stomata, which are outlets from the 

 intercellular passages. Horizontal leaves have usually their spongy 

 tissue and stomata on the lower side, an arrangement which restricts 

 evaporation since the evaporating tissue is not exposed to so much 

 heat. Vertical leaves, have compact palisade tissue on both faces, 

 and all these leaves with thick cuticle have a reserve supply of water 

 stored in the epidermal cells, and sometimes in several layers beneath 

 them ; but these facts can be shown only by means of rather skillful 

 work with the microscope. 



The live oak seems to transpire rather freely, but it is not very 

 active during the summer. In Southern California it simply matures 

 its acorns, then appears dormant for several months ; in earl} 7 spring 

 the new leaves come out with a sudden burst. The Eucalyptus seems 

 never to intermit its activity. The extraordinary avidity of its roots 

 for water makes it useful in lands that need drainage, as well as in 

 arid regions. In some species its habit of vertical leaves comes with 

 increasing years, the leaves on young trees being horizontal, and it is 

 very interesting to watch the transition. Many of our native shrubs 

 have leaves of the same type as the oak, so have some common intro- 

 duced trees, such as the Magnolia and India-rubber ; but the pepper 

 tree, and the many acacias have practically vertical foliage. There is 

 a marked tendency to vertical leaves in other native shrubs besides 

 the Manzanita. Perhaps the fact that many cone-bearing trees do well 

 in our climate with little water should be noted. Their leaves, of 



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