320 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 



the dry sain Is of the plains much narrower leaves than in the valleys of the moun- 

 tainous regions. In conjunction with the narrowing of the foliage, the wrinkling of 

 the leaves has to be considered, i.e. the formation of grooved depressions on the sur- 

 face. Strictly speaking, there is no lessening of the whole surface of the leaf, but 

 only of that portion of the surface which is exposed to sun and wind. This is the 

 point with which we are concerned. With regard to the exhalation of water, only 

 the extent of the surface directly influenced by the agents for increasing transpira- 

 tion is to be considered; whilst the extent of the grooved depressions, which are not 

 exposed to the sun's rays, nor to dry currents of air, may be in a certain measure 

 neglected. On the whole, plants with wrinkled and grooved foliage are not very 

 abundant. For the most part the crumpling is to be seen on quite young leaves 

 when first they break through the bud-scales, and when their epidermal cells are not 

 yet sufficiently thickened with cuticular material. Later, when the formation of the 

 cuticle is advanced, the wrinkles gradually become smooth, and the leaf becomes 

 flat. 



It has already been pointed out that those pit-like depressions, on the floor of 

 which stomata are concealed (cf. figs. 68 and 73), may also serve to restrict trans- 

 piration. There is no contradiction in the statement that the same structure at one 

 time hinders the entrance of water and the wetting of the stomata at the bottom of 

 the pit, and at another time prevents direct contact with dry winds and consequent 

 over-transpiration. Each has its turn. When the foliage of the Australian Prote- 

 aceas, during the summer sleep, is exposed for months to the scorching rays of the 

 sun and to the warm dry air, and when all supplies of water from the soil have 

 ceased, evaporation from the leaves must be restricted as much as possible; it is then 

 that the pit-like depressions perform their duty in this respect. When, later on, the 

 plants are aroused from their long sleep, and have to provide themselves with food, 

 to grow, blossom, and fructify in an extremely short space of time, while violent 

 showers of rain are pouring down from the clouded sky, and all the leaves are 

 dripping with wet; it is then very important that, in spite of these exceedingly 

 unfavourable conditions for evaporation, an abundant transpiration should never- 

 theless take place, and that the function of the stomata should be in no way 

 impaired by the moisture. These pit-like depressions, which in the dry period pre- 

 vented evaporation, now have to keep moisture away from the stomata. 



In many plants evaporation from the superficial tissue is restricted by the close 

 contact of the leaves to their supports, like the scales on the back of a fish. The 

 upper side of a leaf in contact with the stem, and frequently adhering to it, is thus 

 deprived of the means of exhalation, and transpiration can only take place on the 

 somewhat arched or keeled under side of the green scale-like leaf. This occurs, 

 for example, in the Tree of Life (Arbor vita), in several species of Juniper, in 

 Thujopsis, Libocedrus, and various other Conifers. It is not without interest to 

 notice that in several of these Conifers the little green scale-like leaves only become 

 close pressed to the stem when they are exposed to the sun, whilst they project 

 from it if the branches in question are shaded. 



