MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 635 



sink down; the main point is that they turn their profile towards the night sky, 

 and this occurs in all the above-mentioned cases. But it should be noticed here 

 that besides the protection against excessive loss of heat, other advantages are 

 gained by the periodic alteration of the position of the leaflets, and in this respect 

 it is anything but a matter of indifference whether the leaflets fold together 

 above or below. Since the vertical position of the leaf -surfaces also furnishes an 

 important protection against excessive transpiration, various conditions of the leaf 

 construction connected with this protection are also significant. For example, the 

 question whether the stomata are developed on the upper or under side of the 

 leafiet is determined, inasmuch as the sides provided with stomata, as a rule, come 

 in contact with one another. Finally, it must not be denied that bedewing also has 

 an infiuence on the alteration of position of the delicate leaflet. 



A great number of plants whose leaflets assume the sleep position at nightfall 

 also exhibit this phenomenon on bright days when shaken or touched, and indeed 

 more rapidly under these circumstances than at the on-coming of darkness. The 

 slightest touch of the finger, even shaking by a moderate wind, suffices to cause 

 the leaflets of these plants to fold together. In the Oxalis sensitiva of tropical 

 India even the disturbances of the air caused by the approach of man is enough to 

 cause the pinnate leaflets to fall together rapidly, and the same thing occurs in 

 several papilionaceous plants (e.g. Smithia sensitiva and uEschynomene Indica), as 

 well as in several mimosas. When we move away from the immediate vicinity of 

 these plants, and complete stillness once more reigns in the air, the folded leaflets 

 again spread out and turn their upper surfaces skyward. The phenomenon that 

 the plants close up, frightened at the approach of man, as if they felt or in some 

 way became aware of his approach, caused the older botanists to name them 

 Sensitive Plants. 



From a cursory examination it appears that the folding of the leaflets in these 

 sensitive plants caused by shaking, and the assumption of the sleep position at the 

 setting in of darkness, are the same processes; but closer investigation shows that 

 there is an essential difference between them. Outwardly this diff'erence is recogniz- 

 able by the fact that in the sleep position, brought about by darkness, the pulvinus 

 below a leaflet remains quite rigid, while in the folding of the leaflets produced by 

 shaking a relaxation of one half of the pulvinus occurs. In sections through the 

 pulvinus of sensitive plants, it is seen that numerous intercellular spaces are 

 contained in that portion of the parenchyma which adjoins the flexible strand of 

 vascular bundles. It is also seen in these sections that the thickness of the cell- 

 walls in one half of the pulvinus is three times as great as in the opposed half, and 

 that all these cells communicate with each other by extremely fine canals. If the 

 thick-walled portion of a pulvinus is disturbed with a bristle, no alteration is pro- 

 duced; but as soon as that side characterized by its delicate cell-walls is touched 

 ever so lightly, it changes colour. It now appears a darker green, due to the fact 

 that watery sap has been pressed out from the cells into the intercellular spaces. 

 The slightest pressure is felt, accordingly, as a stimulus by the protoplasm in those 



