126 THE BIOLOGY OF FLOWERING PLANTS 



It is legitimate to regard all narrow, more or less erect, 

 leaves, such as those of many grasses, as showing this feature 

 to a certain extent. The panphotometric condition may be 

 assumed temporarily by pulvinate leaves. A leaf arrange- 

 ment which does not receive full illumination is a great deal 

 more common than would at first sight appear. This 

 aspect of the vegetation of our heaths and pastures awaits, 

 and would repay, investigation. 



§ 13. Leaf Structure and Assimilation 



The four pigments which we term collectively chloro- 

 phyll are localised in plasmatic bodies of definite structure 

 and individuality which are called chloroplasts. There are 

 good grounds for believing that the chlorophyll is present 

 in these in the form of a colloidal solution. Among the 

 algae we meet with great variety in the form, size, and number 

 of these speciahsed plastids. From the mosses upwards, 

 with rare exceptions, many small, thin, elliptical chloroplasts 

 occur in each cell. In the typical dorsi-ventral leaf they are 

 most numerous in the palisade parenchyma, next the upper 

 surface. In leaves of the bifacial type, more or less typical 

 pahsade tissue abuts on both surfaces. Haberlandt has 

 described the various types of palisade tissue. Its chief 

 feature is the regular elongation of the cells at right angles 

 to the surface of the leaf, so that the light strikes down 

 through a series of tubes lined with green plastids. These 

 cylinders are interspersed with narrow intercellular spaces. 

 Below the palisade lies the spongy parenchyma with fewer 

 chloroplasts and larger air spaces, through which the carbon 

 dioxide passes to the palisade tissue. Frequently there 

 can be made out a definite relation between groups of the 

 palisade cells and cells of the spongy parenchyma, leading 

 to efficient transport of the sugar formed in photosynthesis, 

 or produced at night by the hydrolysis of the starch stored 

 in the plastids. The chloroplasts line the walls of the 

 palisade cells, and more particularly the vertical elongated 

 walls. When a cross wall is bounded to the outside by an 



