LEAVES 569 



weak development of cutin. Submersed plants, both in fresh and in 

 salt water, commonly are free from cutinization. 



The influence of external factors upon cutinization. — No plant struc- 

 ture reacts more readily to changes in conditions than does the cuticle, 

 submergence in water inhibiting its formation, and desiccation favoring 

 its maximum development. In the air the thickness of the cuticle 

 appears to vary directly with the transpiration, it being thinner on under 

 than on upper leaf surfaces, thinner in stomatal pits than at the surface 

 (figs. 801, 807), and thinner in protected than in exposed situations, as 

 in the basal leaves of Tilia in moist woods, in comparison vidth its top 

 leaves on a dry hill (see figs. 770, 771). As in the case of palisade cells, 

 cutin formation approaches its maximum, both where transpiration is 

 large in amount, as in most xerophytes or even in exposed hydrophytes 

 like the bulrush, and where there is a high ratio of transpiration to 

 absorption, as in alpine and arctic habitats, in peat bogs, and in tropi- 

 cal salt marshes. Cutin formation is increased when ordinary meso- 

 phytes (such as wheat) are grown in concentrated solutions, and in 

 the mangrove, cutinization is most marked in the saltiest soils. 



In at least one submersed marine plant, Cymodocea, the epidermis is cutinized, 

 and the nearly related Zostera has no epidermal chlorophyll; it is possible that 

 these features of air leaves are due to the high concentration of the sea water. It 

 should be noted that the cuticle is not always a plastic structure; in the conifers it 

 seems as rigid and as unrelated to environment as are any of the internal tissues. 



The role of cutin. — Cutin retards the egress of water from leaves, 

 not so much because of its thickness, as because its fatty character 

 makes it relatively impermeable to water. The transpiration from a 

 peeled apple for a period of three hours is twenty times that from an 

 apple with cuticle intact. A water leaf, exposed to dry air, withers 

 almost immediately because of its uncutinized epidermis. If the 

 stomatal surface of a Ficus leaf is coated with wax, the loss of water is 

 enormously reduced, and may amount in one day to but one two- 

 hundredth of that from a water surface of equal area. The cuticle, 

 therefore, is a transpiration-reducing structure of high efficiency, and 

 were it not for the stomata, which entail abundant evaporation, leaves 

 would be almost perfectly protected by their cutin layer. Its value is 

 apparent, not only in dry habitats, but also in peat bogs and in salt 

 marshes, as well as in alpine and arctic conditions, because of inade- 

 quate absorption. 



