THE CAUSES OF PALISADE DEVELOPMENT 



43 



means exhausts the Hst. One of these is a leaf in which there is 

 palisade both above and below with some spongy ])arenchyma be- 

 tween. This is most likely to be found in leaves that have a vertical 

 or nearly vertical orientation so that the two sides of each leaf are 

 about equally exposed. A second type is that found in plants with 

 succulent, or fleshy, leaves. In these the chlorenchyma is often 

 reduced to a few layers of cells and the larger part of the leaf tissue is 

 made up of colorless water storage 

 cells. This tissue is sometimes next 

 the epidermis and in other cases oc- 

 cupies the central portion of the leaf. 

 A convenient term that is often used 

 to designate all of the internal tis- 

 sues of a leaf, with the exception 

 of the veins, is mesophyll. Thus in 

 a succulent leaf the mesophyll is 

 partly chlorenchyma and partly 

 water storage tissue. In Arctic re- 

 gions and at high elevations on 

 mountains there is a third common 

 type of leaf in which the mesophyll 

 is made up entirely of chlorenchyma Fig. 17.— Section of a leaf of a 

 which can be said to be neither desert plant, mesquite iProsopis 



. -n 1 veliitina), UE, upper epidermis; P, 



palisade nor spongy tissue. Kather paUsade tissue; SP, spongy tissue; 

 it is a very compact parenchyma LE, lower epidermis; S, stoma, 

 tissue of rounded or angular cells. 



25. The Causes of Palisade Development.— The two factors that 

 have usually been thought to have most influence on palisade de- 

 velopment are light and transpiration. The amount of palisade 

 tissue often varies considerably according to the relative exposure of 

 the plant. A plant of wild lettuce {Laduca scariola), for example, 

 which has grown in a fully exposed place has relatively thick leaves, 

 the mesophyll of which is all palisade tissue, while a plant of the 

 same species, which has grown in deep shade, may have leaves only a 

 third as thick as the former plant and with scarcely any palisade 

 tissue. These different types of leaves from the same species are 

 spoken of as sun leaves and shade leaves respectively. In the case 

 of trees, such as the maple, it is often possible to collect leaves from 

 the same individual plant which have the structural characteristics 

 of typical sun and shade leaves; the sun leaves being taken from the 

 south side of the tree where they are fully exposed to light and the 



