ADAPTATIONS TO REDUCE TRANSPIRATION. 431 



ing Smilax waiter i), have leaves that are strongly bifacial (dorsiven- 

 tral) as to structure, and plagiotropic in position, i. e., approximately 

 horizontal, and thus receiving the incident rays as nearly as may be 

 at a right angle. As has already been mentioned, in discussing adap- 

 tations to environment in the Sand Strand, leaves thus constituted 

 and placed, unless shaded by other plants or objects from the direct 

 rays of the sun, may have their structure modified in various ways so 

 as to reduce the amount of water transpired, while some of the modifi- 

 cations may serve likewise to shield the sensitive chloroplasts from 

 the injury and even decomposition caused by excess of light. Con- 

 sequently we find the following arrangements. 



2. Position of the stomata.—ln all the species examined (excepting 

 Phoradendron and Smilax ira/teri, the latter to a certain extent shade- 

 loving) the stomata occur exclusively on the lower (dorsal) leaf sur- 

 face, and are thus protected by the whole thickness of its tissues from 

 the direct light and heat rays. The stomata of Smilax laurifolia are 

 further protected by being deeply sunken, the entire thickness of the 

 massive cuticle lying above the guard cells, while exchange of gases 

 with the outer air is permitted only by a narrow canal through the 

 cuticle. 



3. Epidermal outgrowths.— The lower surface of the leaf is pro- 

 tected in some species by outgrowths of the epidermis. 



(a) Hairs, which soon lose their living contents and become filled 

 with air. These are in some cases sufficiently numerous to form a 

 close, downy covering on the under surface of the leaves, especially 

 when young. * They occu r in the following species : Persea pubescens, 

 Acer rubrum var., Magnolia virginiana, Nyssa uuiflora. This hairy 

 covering, by forming spaces between the hairs and the leaf surface 

 which are not readily accessible to atmospheric currents, undoubtedly 

 assists in reducing the quantity of vapor of water transpired from the 

 leaf. It may also help to keep the leaf surface free from rain water, 

 which would otherwise cover the mouths of the stomata at times and 

 hinder the entrance and exit of gases. A number of species with per- 

 sistent, leathery leaves, i. e., Ilex glabra, I. lucida, Leucothoe axil- 

 laris, have scattered, prickle-like hairs, which are confined to the 

 impressed larger veins on the ventral surface of the mature leaf. 



(6) A coating of wax, giving the leaf surface a glaucous appear- 

 ance. This occurs in Acer rubrum, 2 Magnolia virginiana, Persea 

 pubescens, Rosa Carolina, Kahnia angustifolia. In Berchemia scan- 

 dens the cuticle of both surfaces of the strongly ombrophobic (water 

 shedding) leaf is granular-roughened, probably with a slight deposit 



1 In very young leaves the upper surface also is protected. 



2 The red maple when growing in sunny situations, as on the margin of Lake 

 Drummond, has a thickish leaf, with dark green, shining upper surface, and a 

 very glaucous, usually more or less pubescent, dorsal surface. In deep shade the 

 leaf is thin, and merely pale or only slightly glaucous, usually not pubescent 

 beneath when mature. 



