PROTECTION FROM EXCESSIVE TRANSPIRATION 49 



within the plant is so vital that the possibility of losing it faster than 

 it can be absorbed is an ever present and very real danger. For 

 this reason we find a variety of structural modifications that appear 

 to be designed to prevent excessive transpiration. 



The stomatal movements described in paragraph 28 tend to control 

 the rate of transpiration. In many cases, also, the stomata them- 

 selves are protected in one way or another. Sometimes they are 

 merely sunken below the level of the epidermis, which protects them 

 from air currents. Sometimes, as in the oleander, several stomata 

 are grouped within a pocket-like depression which is clothed with 

 hairs. Again, they may have extra well developed outer and inner 

 vestibules, or a labyrinthiform passageway in place of the simple 

 slit. Any of these devices tend to retard the exit of water. 



In practically all leaves that are exposed to the air the epidermis 

 itself is covered with a layer of cutin which renders it so nearly 

 water-proof that transpiration is confined almost entirely to the 

 stomata. Sometimes the cutin is reinforced, or in some cases 

 replaced, by a coating of wax or of resin. This not only retards 

 transpiration directly but also tends to check rapid heating of the 

 leaf, which in turn influences the rate of transpiration. In the case 

 of floating leaves, such as those of the water lily, the waxy covering 

 has another use in that it prevents the surface from becoming wet. 

 This facilitates the interchange of gases for respiration and photo- 

 synthesis by preventing the stomata from becoming rtogged with 

 water. 



The surfaces of many leaves, as well as stems and other plant 

 parts, are often covered with hairs. These may be simple or 

 branched. Usually the simple hairs stand perpendicular to the 

 surface but in a few cases, as on the bud scales of Norway maple, 

 they are closely appressed, or, in other cases, as on the stems of 

 Florida moss {TiUandsia), they are scale-like and overlap like the 

 shingles of a roof. The common mullein {Verhascmn thapsus) has 

 multicellular hairs that are much branched and form a woolly cover- 

 ing of the leaves. These various kinds of hairs have been thought 

 to be rather efficient organs of protection against excessive trans- 

 piration. In many cases they doubtless do afford some protection, 

 especially in windy weather, but exi^eriments and observations that 

 have been made seem to indicate that their efficiency in this respect 

 has often been over-emphasized. 



One of the most efficient means of protection against excessive 

 loss of water is by reducing the transpiration surface. iNIost efficient 

 4 



