IRRITABILITY. 591 



it receives as much light as possible of an intensity sufficient 

 for the performance of the constructive metabolic processes, 

 it should receive as little light as possible of such an intensity 

 as to lead to the decomposition of its chlorophyll. The 

 fixed light-position of a leaf is then the position in which 

 both these ends are as far as possible attained, the position 

 being different in leaves of different organisation. 



In connexion with this subject we will briefly consider 

 the peculiarities of structure of leaves which are involved 

 by the nature of the fixed light-position of leaves (see p. 

 384). A leaf which when fully exposed to light takes up 

 such a position that its morphologically upper surface is 

 directed towards the zenith, presents a well-marked difference 

 in its internal structure between the upper and lower por- 

 tions of its mesophyll. Towards the upper surface the meso- 

 phyll consists of elongated cells closely packed together, with 

 scarcely any intercellular spaces, the long axes of the cells 

 being perpendicular to the surface : this is the pallisade- 

 tissue. Towards the lower surface the mesophyll consists 

 of irregularly shaped cells with large intercellular spaces : 

 this is the spongy tissue (see Fig. 13, p. 70). Moreover there 

 are but few stomata, if any, in the epidermis of the upper 

 .surface, whereas they are numerous in that of the lower. 

 Such a leaf is manifestly dorsiventral. A leaf which when 

 fully exposed to light takes up such a position that its 

 surfaces lie in a vertical plane, does not exhibit this structure. 

 At both surfaces of the leaf pallisade- tissue is developed, 

 usually to an equal extent, with a similar distribution of 

 intercellular spaces, and stomata are present in the epidermis. 

 Such a leaf is isobilateral. The difference in the anatomy of 

 the two leaves is due to the fact that in the case of the 

 horizontal leaf the one surface, the upper, is more directly 

 exposed to light than the lower, whereas in the case of the 

 vertical leaf both surfaces are equally exposed to light. 

 Now the particular kind of tissue in which the processes 

 of constructive metabolism can be most advantageously 

 carried on is the spongy parenchyma : this kind of tissue 

 most readily permits the entrance of light into the tissues. 



