FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 339 



nected with the nocturnal radiation of heat (as will be explained later) and not with 

 exhalation. It is, however, equally certain that the placing together and folding up 

 of leaves and leaflets in many other plants is brought about in order to prevent 

 over-transpiration and consequent withering up. Many shrubby, thorny mimosas 

 of Brazil and Mexico, when in their native habitat and position, extend their leaflets 

 horizontally when evening approaches, contrary to the behaviour of the well-known 

 Sensitive Plant (Mimosa pudica), and they remain in this position throughout the 

 night. Next morning they are still widely outspread. As soon as the sun has 

 risen, and its beams fall on the foliage, the leaflets shut together; the menacing 

 thorns, which until now have been hidden by the extended leaves, become 

 apparent; and the leaflets remain in the vertical position during the hottest and 

 driest hours of the day. Towards sunset they again rise and are extended 

 horizontally. There is but one exception to this cycle of changes— if the opened 

 leaf is shaken by the wind, and if the sky has been gray and clouded all day. In 

 the former case, under the influence of the wind, a rapid closure occurs; in the 

 latter case, when the weather is bad, they remain open all day: One of the 

 Rutaceae, Porliera hygrometrica, behaves like these mimosas. In Peru, the native 

 country of these plants, where they abound, the opening and closing of the leaves 

 has even been made use of for weather predictions, for when the vertical leaves are 

 closed, dry hot weather can be reckoned upon; when they are open, damp cool 

 weather. In the cultivated Bean (Phaseolus), moreover, alterations of position in 

 parts of the leaflets may be observed to take place during the day. When the sun 

 is powerful, the leaflets assume a vertical position, so that at noon the sun's rays 

 only reach a small portion of the blade. 



In several species of Wood-sorrel belonging to the South African flora, and 

 also in the widely-distributed Common Wood-sorrel (Oxalis Acetosella), it may be 

 noticed that the leaflets, as soon as they are directly struck by the sun's rays, sink 

 down, so that their under surfaces — qn which the stomata are situated — face one 

 another, the three leaflets together forming a pyramid; while these same leaflets 

 in damp shady places remain extended. The leaflets of the water fern, Marsilea 

 quadrifolia, which grows in marshes and is distributed through Central and 

 Southern Europe, temperate Asia, and North America, are very similar to those of 

 the Wood-sorrel, but carry their stomata on the upper surface. As long as they 

 remain floating on the surface of water, these leaflets are extended, but as soon as 

 the water-level sinks and the leaflets become surrounded by air, they fold together 

 above in the sunshine, and their position becomes vertical, precisely as in the 

 compass plants. 



As another phenomenon of this kind the periodic folding or closing of the leaves 

 of grasses must be specially mentioned. It has long been noticed that certain 

 grasses exhibit a very different aspeet according as they are observed on a dewy 

 morning or in the noon-day sunshine. In the morning their long linear leaves are 

 fluted on the upper surface, or spread out quite flat. As soon as the humidity of 

 the air diminishes, in consequence of the higher position of the sun, they fold 



