ADAPTATION TO LIGHT 



183 



forests, in which it is a rehc of the structure of the ancestral sun 

 leaf. 

 * Isophotic leaves fall into three types, based upon the intensity 

 of fhe light. The palisade leaf, or staurophyll, is a sun leaf in 

 which the chlorenchym consists wholly of rows of palisade cells. 

 It is produced by nearly equal illumination of both surfaces, due to 



Fig. 61. Isophotic leaf (1) of an oak, Quercus novimexicana. The shade 

 leaf (2) is of the same type, though one row of pahsade tissue is lost 

 and the leaf is thinner. 



its upright position or to reflection from a light-colored soil. In a 

 special form of this t}T)e, the diplophyll or double leaf, the intense 

 light does not penetrate to the middle of the leaf. In consequence, 

 the upper and lower palisade areas are separated by a central 

 sponge-like tissue, which is used for the storage of water. The 

 sponge leaf, or spongophyll, includes all shade leaves, except those 

 in which some palisade persists from the ancestral sun form, and 

 practically all submerged leaves. Its chlorenchym consists of 

 sponge-cells alone. Certain monocotyledons, which grow in the 

 sun but lack palisade, may also be referred to this type for the 

 present. 



199. Sun plants and shade plants. Sun plants are also termed 

 helioplu/ies and shade plants sciophytes. The former comi^rise 

 practically all xerophytes, prairie and meadow mesophytes, and 

 amphibious and floating h}-drophytcs. Shade plants include 

 'the mesophytes of thickets and forests and submerged hydro- 

 phytes. The differences in the form of stem and leaf shown by 

 these two types have already been discussed. The greater number 



