THE OUTER TISSUES OF PLANTS. 263 



From the ordinary cases let us now pass to the exceptional 

 cases. We will look first at those in which the two faces of 

 the leaves differ but little, or not at all their circumstances 

 being similar or equal. Leaves that grow in approximately- 

 upright attitudes, and attitudes which do not maintain the 

 relative positions of the two surfaces with constancy, may be 

 expected to display an unusual likeness between the two 

 surfaces; and among tnem we see it. The Grasses may be 

 named as a group exemplifying this relation; and if, instead 

 of comparing them as a group with other groups, we compare 

 those dwarf kinds of them which spread out their leaves 

 horizontally, with the large aspiring kinds, such as Arundo, 

 we trace a like antithesis : in the one the contrast of upper 

 and under is very obvious, while in the other it is scarcely to 

 be detected. Leaves of various other Monocotyledons that 

 grow in a similar way, similarly show us a near approach to 

 uniformity of the two surfaces ; as instance the genus Olivia 

 and the thinner-leaved kinds of Yucca. Where the con 

 trast of upper and under is greatly diminished by the assump 

 tion of a rounded or cylindrical form, instead of a flattened 

 form, the same thing happens. The genus Kleinia furnishes 

 illustrations. It may be remarked, too, that even within 

 the limits of this genus there are instructive variations ; for 

 while in Kleinia ficoides the leaves, shaped like pea-pods, 

 are broadest in a vertical direction, and have their lateral 

 surfaces alike in conditions and structure, in other species 

 the leaves, broader horizontally than vertically, exhibit 

 unlikeness between the upper and under sides. Equally 

 to the point is the evidence furnished by vertically-growing 

 leaves that are cylindrical, as those of Sanseviera cylindrica, 

 or as those of the Rush-tribe: the similarly-placed surface 

 has all around a similar character. Of kindred moaning, 



other varieties. Chlorophyll, then, must be regarded simply as the most 

 general of the colouring matters found in those parts of plants in which 

 assimilation is being effected by the agency of light. Though it is always 

 present alonn with the red and brown pigments, yet there is much evidence to 

 show that these are the actual assimilative pigments. 



