STEM HABITS 33 



by a deposit of suberin. The efficiency of suberin as a water-proofing 

 substance is well known through the use of commercial cork, ob- 

 tained from the bark of the cork oak, for bottle stoppers and many 

 other purposes. Just as in the development of cuticle, the chief 

 factor in the formation of cork seems to be desiccation. The initial 

 step in cork formation is probably always a cutinization or suberiza- 

 tion of walls, usually in the presence of air, which blocks the flow of 

 sap and results in an accumulation of food. This accumulation of 

 food brings about the formation of a growing tissue called a cork 

 cambium which produces the cork tissue. 



21. Stem Habits.— In plants with foliage the greatest loss of water 

 takes place through the leaves. For this reason the various habits 

 of growth and of foliage display result in varying degrees of protec- 

 tion from excessive loss of water. From this point of view woody 

 plants may be classified as follows: 



(a) Tropical Evergreen Trees and Shrubs.— These are the only 

 plants that can be said to have unrestricted foliage display. They 

 grow in the rain forests of the tropics where the uniform climate and 

 ever abundant moisture make protection unnecessary. The condi- 

 tions for growth in such a place are in many respects ideal and the 

 vegetation is very luxuriant. The stems of the bamboo which grow 

 in these forests are among the most rapid growers known. They 

 sometimes grow nearly a foot in one day. On the other hand the 

 great luxuriance of the vegetation brings about an intense compe- 

 tition for space. 



(6) ScleropJiyllous Evergreen Trees and <S^rM6*.— Sclerophyllous 

 evergreens are those in which the epidermis of the leaves is composed 

 of thick walled (sclerenchyma) cells with a very heavy cuticle. Such 

 an epidermis makes the leaf stiff and leathery. Sclerophyllous plants 

 are of two general tjqDes. In regions having hot dry summers and 

 warm wet winters, as in some parts of southern California, the sclero- 

 phyllous vegetation has broad flat leaves. Examples of this type are 

 the holly, the live oak and the olive (Fig. 11). In regions with cold 

 winters, on the other hand, we find sclerophyllous plants with the 

 familiar needle-shaped leaves of the pines and spruces. In either 

 case the advantage of the sclerophyllous habit is* that with the pro- 

 tection afforded by the heavy epidermis these plants are able to dis- 

 play their foliage at all times and so to carry on photosynthesis 

 throughout the year. At the same time there is the disadvantage 

 that this type of leaf, which effectually reduces the rate of transpira- 

 tion, also reduces the rate of gas exchange and so the rate of 

 3 



