338 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 



position, but that the broad surfaces of each leaf always faced the rising and setting 

 sun. Healthy living plants as they grow in the sunny meadows look as though 

 they had been laid between two gigantic sheets of paper, somewhat pressed, and 

 dried for some time in the way plants are prepared for herbariums, and had then 

 been removed from the press and set up so that the apex and profile of the vertical 

 leaf-blades point north and south, i.e. in the meridian; while their surfaces face 

 the east and west. This inclination is so well and regularly observed by the living 

 plants on the prairies, that hunters are enabled to guide themselves over such 

 regions, even under a clouded sky, by means of these plants; for this reason Sil- 

 phium laciniatum has been called a "compass" plant. The life of the compass 

 plant is assisted by this placing of the vertical leaves in the meridian, in that the 

 broad surfaces are placed almost at right angles to the incident sunbeams which 

 illuminate them in the cool and relatively damp morning and evening, while at the 

 same time they are not too strongly heated nor stimulated to excessive transpiration. 

 At mid-day, on the other hand, when the sun's rays only fall on the profile of the 

 leaves, the heating and transpiration are proportionately slight. It is of interest 

 that the leaves of these compass plants, as well as those of the above-mentioned 

 Lettuce represented with the compass plant in fig. 84, show this inclination and 

 position when they grow on level, moderately dry, unshaded ground, and that in 

 damp shady places, where there is no danger of over-transpiration from the 

 powerful rays of the noon-tide sun, the twisting of the leaves does not take place, 

 and they are not brought into the meridian. 



The placing of their leaf-blades parallel to the ground when in the shade, but 

 vertically when in dry sunny places, is, generally speaking, a phenomenon which 

 may be seen in very many plants, including shrubs and trees. A species of lime, a 

 native of Southern Europe, viz. the Silver Lime (Tilia argentea), is particularly 

 noticeable in this respect. On dry hot summer days the leaves assume an almost 

 vertical position, but only on those boughs and twigs which are exposed to the sun. 

 If the tree stands at the foot of a wall of rock, or on the edge of a thick wood, so 

 that a portion of it is shaded, the leaves on this shaded part remain extended 

 horizontally. Such a tree then presents a strange aspect, as the leaves are of 

 two colours — dark green on the upper side, and white on the under surface by 

 reason of a fine felt-work of white stellate hairs — and it is scarcely credible at first 

 sight that the shaded and sunny portions of the tree belong to one another. 



In the compass plants and also in the Silver Lime the alterations in the 

 direction of the leaves are brought about by alterations in the turgidity of certain 

 groups of cells in the leaf-stalk. It is exactly the same cause which produces the 

 periodic movements of numberless plants with pinnate or palmate leaves, and the 

 leaf-folding of many grasses; and it is natural to conjecture that these phenomena 

 of movement are also connected with transpiration. This is in part actually the 

 case. In consequence of alterations in turgidity of the pulvini, the pinnate leaflets 

 of the Gleditschias and some Mimosas rise up after sunset, while those of the Amor- 

 phas fall down, and assume a vertical position during the night; but this is con- 



