394 RELATION TO ENVIRONMENT. 



the hairs form a felt-like covering as in the common mullein 

 some antennarias, etc., they lessen the loss of water vapor be' 

 cause the air-currents close to the surface of the leaf are retarded. 

 Spines (see the thistles, etc.) also afford a protection against 

 certain animals. 



765. (3) Reduction of surface. Reduction of leaf surface is 

 brought about in a variety of ways. There are two general 

 modes: (ist) Reduction of surface along with reduction of 

 mass; (2d) Reduction of surface inversely as the mass. Ex- 

 amples of the first mode are seen in the dissected leaves of many 

 aquatic plants. In this finely dissected condition the mass of 

 of the leaf substance is much reduced as well as the leaf surface, 

 but the leaf is less liable to be injured by movement of the water. 

 In addition it has already been pointed out that lobed and 

 divided aerial leaves are much less liable to injury from violent 

 movements of the air, than if a leaf with the same general out- 

 line were entire. The needle leaves of the conifers are also 

 examples, and they show as well structural provisions for pro- 

 tection in the thick, hard cell- walls of the epidermis. To off- 

 set the reduced surface there are numerous crowded leaves. 

 Reduction of surface inversely as the mass, i.e., the mass of 

 the leaf may not be reduced at all, or it may be more or less 

 increased. In other words, there is less leaf surface in pro- 

 portion to the mass of leaf substance. It is probable in many 

 cases, example: the crowded, overlapping small scale leaves of 

 the juniper, arbor vitae, cypress, cassiope, pyxidanthera, etc., that 

 there has been a reduction in the size of the leaf, and at the 

 same time an increase in thickness. This with the crowding 

 together of the leaves and their thick cell-walls greatly lessens 

 the radiation of moisture and heat, thus protecting the leaves 

 both in dry and cold weather. The succulents, like " live-for- 

 ever," have a small amount of surface in proportion to the mass 

 of the leaf. In the yucca, though the leaves are often large, 

 they are very thick and expose a comparatively small amount 

 of surface to the dry air and intense sunlight of the desert regions. 

 The epidermal covering is also hard and thick. In addition, 



