FOLIAGE LEAVES. 391 



larger veins replaced by stems bearing simple leaves. The tree 

 as it is, however, has the advantage of being able to cast off for 

 the winter period a layer of what otherwise would have been a 

 portion of the stem system, to retain which through the winter 

 would use more energy than with the present reduced stem 

 system, and the stouter stem is less liable to dry out. In the 

 case of herbaceous plants, in the case of plants like most of 

 the ferns where the stem is on the underground rootstock (Pteris) , 

 or a very short erect stem, as in the Christmas fern, the leaf 

 replaces the aerial stem, and the division (or branching, as it is 

 sometimes styled) of the leaf corresponds to the branching of the 

 stem. This is more marked in the gigantic exotics like Cibo- 

 tium regale, and in the tree ferns which have quite tall trunks, 

 the massive compound leaves replace branches. In the palms 

 and cycads are similar examples. Those who choose to observe 

 can doubtless find many examples close at hand. (3) While 

 divided leaves have probably not been evolved in response to 

 the light relation, still their relation in this respect is an impor- 

 tant one, since if the leaf with its present size were entire, it 

 would cast too dense a shade on other leaves below. 



760. General structure of the leaf. The general structure of the leaf 

 has been already studied (see Chapters IV, V, VII). It is only necessary 

 to recall the main points. The upper and lower surfaces of the leaf are 

 provided with a layer of cells usually devoid of chlorophyll. The mesophyll 

 of the leaf consists usually of a layer of palisade cells beneath the epider- 

 mis, and the remainder consists of loose parenchyma with large intercel- 

 lular spaces. Through the mesophyll course the "veins," or fibro-vas- 

 cular strands, consisting of the xylem and phloem portions and serving 

 as conduits for water, salts, and foodstuffs. In the epidermis are the 

 stomata, each one protected by the two guard cells. The guard cells as 

 well as the mesophyll contain chlorophyll. The stomata and the com- 

 municating intercellular spaces furnish the avenues for the ingress and 

 egress of gases, and for the escape of water vapor. 



761. Protection of leaves. There are many modifications of the general 

 plan of structure in different leaves, many of them being adaptations for 

 the protection of the leaf under adverse or trying conditions. Many 

 leaves are also capable of assuming certain positions which afford them 

 protection. The discussion of this subject may be presented under two 

 general heads: Protective modifications; protective positions. 



