THE OUTER TISSUES OF PLANTS. 247 



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 Endogens that grow 

 in a similar way, similarly show us a near approach to uni- 

 formity 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 as- 

 sumption of a rounded or cylindrical form instead of a flat- 

 tened 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 ficoidcs 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 t or as those of the Eush- tribe : the similarly-placed 

 surface has all around a similar character. Of 



kindred meaning, and still more conclusive, are the cases in 

 which the under side of the leaf, being more exposed to 

 light than the upper side, usurps the character and function 

 of the upper side. If a common Flag be pulled to pieces, 

 it will be seen that what answers to the face in other 

 leaves, forms merely the inside of the sheath including the 

 younger leaves, and is obliterated higher up. The two sui 

 faces of the blade answer to the two under halves of a 

 leaf that has been, as it were, folded together lengthways, 

 with the two halves of its upper surface in contact. And 

 here, in default of an upper surface, the under surface acquires 

 its character and discharges its function. A like substitution 

 occurs in Witsenia corymbosa ; and there are some of the 

 Orchids, as Lockhartia, which display it in a very obvious way, 



