532 MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 



This opening and closing of the cotyledons is seen particularly well in species of 

 clover and Bird's-foot Trefoil (Trifolium and Lotus), in all mimosas and bauhinias, 

 and numerous other leguminous plants; also in species of wood sorrel (e.g. Oxalis 

 Valdiviana, rosea, sensitiva), in the gourds, cucumbers, and melons, in the Sun- 

 flower (Helianthus annuus) and in the Tomato (Solanum Lycopersicum), in 

 species of Mimulus and Mirabilis, the Corn-cockle (Agrostemma Githago), the 

 Chickweed (Stellaria media), and many others. 



By alterations of position, similar to those exhibited by cotyledons, the so-called 

 compound leaves are also in many instances protected against nocturnal radiation. 

 By compound leaves are understood those which bear either pinnate or radiating 

 leaflets on a common stalk. 



These compound leaves in some cases, which have already been alluded to, are 

 spread out during the mild night, but are, on the contrary, folded together under the 

 burning noonday sun. In by far the greater number of cases, however, especially 

 in species whose habitat is exposed to great cooling in the night, the reverse is 

 observed. In sunshine the surfaces of the leaflets are arranged more or less 

 parallel to the ground, the upper side is turned to the sky, and is fully and 

 completely flooded by the sun's rays. If this position were retained after sunset, 

 the surfaces of the leaflets would be forced to give up much heat by radiation 

 towards the night sky. In order to avoid this the leaflets fold together either 

 upwards or downwards, and place themselves, so to speak, on edge. In this way 

 their broad sides become vertical, in which position they are protected from 

 radiation as much as possible. 



There are provided for the accomplishment of these movements certain swollen 

 cushions of succulent tissue at the bases of the several leaflets, and often at the 

 base of the common petiole. These are known as pulvini and each consists of 

 parenchymatous thin-walled cells surrounding a strand of compressed vascular 

 bundles, which further up becomes the midrib of the leaflet, which is inserted on 

 the pulvinus. The parts of this strand where surrounded by the pulvinus are 

 supple and very flexible, but above the pulvinus they become stiff and firm, 

 forming as it were the main support of the whole leaflet, so that indeed alterations 

 of position of the midrib are participated in by the whole. 



In order to represent clearly how a movement is brought about in the leaflet by 

 means of its supporting pulvinus, let us imagine a straight rod which is only flexible 

 at the base, and is there held fast between two springs. The pressures proceeding 

 from the two springs is equally strong, and the rod is therefore maintained in an 

 upright position. If the pressure of the spring relaxes on one side, the stick must 

 bend over in the direction of the diminished pressure. If the pressure of the two 

 springs be afterwards equalized, the rod will again assume its original erect position. 

 If for the rod we now substitute a leaflet traversed by a rod-like midrib, i.e. by the 

 vascular bundle-strand mentioned above, and imagine two opposed halves of a 

 turgid cell-tissue instead of the two springs, then the leaflet will be kept upright 

 by the equal tension of the pulvinus situated at the base of the strand; but as soon 



