384 LECTURE XVI. 



ever, that all young dorsiventral leaves are epinastic at a 

 certain stage in their development (p. 366). The natural 

 unstrained explanation seems to be that the leaves were in the 

 epinastic stage of development, and that when they resumed 

 growth under the tonic influence of light they exhibited 

 epinasty. It is quite possible that, if exposed to light at the 

 appropriate stage of their development, leaves would similarly 

 exhibit a " photo-hyponasty." 



We will conclude our consideration of the relation of 

 leaves to the tonic influence of light with a brief comparative 

 account of their structure when normal and etiolated. In 

 normal leaves, as a rule, the tissue underlying the morpho- 

 logically superior (dorsal) surface of the leaf consists of closely- 

 packed elongated mesophyll-cells so placed that their long 

 axes are at right angles to the surface ; these cells, of 

 which there may be several layers, constitute the pallisade- 

 parenchyma (see Fig. 13, p. 70). The tissue near the morpho- 

 logically lower (ventral) surface consists of irregular loosely 

 arranged mesophyll-cells, with large intercellular spaces, con- 

 stituting the spongy parenchyma. The epidermis of the lower 

 surface is much more abundantly supplied with stomata than 

 that of the upper. The difference of structure between the 

 two surfaces is induced by light. Stahl has shewn that the 

 development of the pallisade-parenchyma is always more 

 marked in leaves which have been fully exposed to the sun 

 than in those which have grown in the shade. It is only 

 when the leaf, as is commonly the case, lies more or less nearly 

 horizontally, that the pallisade-parenchyma is developed ex- 

 clusively towards the upper surface ; when, as is sometimes 

 the case, the leaf-blade lies in a vertical plane, the pallisade- 

 parenchyma is almost equally developed in relation with 

 both surfaces, for both are then exposed to nearly the same 

 illumination. In etiolated leaves the differentiation of pal- 

 lisade-parenchyma and spongy parenchyma does not take 

 place. 



Growth in continuous darkness leads to various other 

 important modifications in the general habit and structure 

 of a plant. In illustration of this let us consider two Potato- 



