264 The Living Plant 



brightness of the sun, with the full exposure to its less intense 

 action at morning and evening. Such a vertical position of the 

 green surface is coimnon in plants of open bright places,— in 

 some, notably certain clover-like kinds, as a temporary and 

 irritably-adjustable position of the leaflets (figure 78), but in 

 others as a permanently vertical arrangement of the leaves. In 

 the most perfect of the latter cases, all the leaf-blades present 

 their faces to the east and the west, thus bringing their edges 

 north and south; and such is the real meaning, and the reason 

 for the name, of the Compass Plants, of which the most perfect 

 and famous example occurs on our own western prairies. In 

 some kinds, instead of the leaf-blade it is the petiole which is 

 flattened and set vertically, the blade being suppressed, as in 

 most of the Australian Acacias (figure 21). In others, advantage 

 is taken of the naturally vertical position of the stem, the function 

 of foliage being transferred thereto from the leaves which are 

 simultaneously reduced or abandoned. This is the case with the 

 Cactuses and innumerable other plants of the deserts, which 

 sometimes acquire additional vertical green surface by the de- 

 velopment of longitudinal ribs. The readiness with which the 

 green tissue can be developed in one part of the plant as well as 

 another helps, by the way, to explain some of the curious mor- 

 phological overturnings represented by plants like the Butcher's 

 Broom (figure 23), or, still better, the familiar Smilax of the 

 florists, in which the apparent leaves are in reality branches, 

 while the actual leaves are no more than tiny scales just beneath 

 them. It is easy to understand that if plants of the desert have 

 once transferred their chlorophyll to their stems, simultaneously 

 suppressing or abandoning their leaves, and then a change of 

 clmiate, or migration to a moister region, should require a larger 

 spread of green surface, this would more easily and naturally be 

 secured through a further flattening of the stems or branches than 

 through a restoration of the lost leaves; and with time such 

 branches would become more and more leaf-like even to the 



