Afc I Hot ns alba, L. and other Plants. 69 



PI. IX, Fig. 4. In this case the photograph was taken directly 

 from the south. On a hot, dry day all the leaves still follow 

 the course of the sun up to as late as six o'clock, as seen in 

 PI. X, Fig. 4, photographed from the north at that hour. 



The preceding will show what is meant by the hot sun posi- 

 tion of leaves. Of the many plants which may be classed 

 with Melilotns in possessing this movement, not all are as 

 extremely sensitive, for all do not possess sensitive pulvini, 

 which render such movements easy. Not a few plants move 

 their leaves into a single hot sun position, in which they re- 

 main during the heat of the day. Others give their leaves a 

 position which will cause them to receive the least of the 

 sun's rays at the hottest time of the day, and then remain 

 rigid from this time on. (Species of Clicnopodium, Sniilax, 

 Laguncularia and others.) 



It is generally supposed by plant physiologists and those 

 who discuss this question, that these movements are protect- 

 ive in character, and that they shield the chlorophyll from too 

 intense illumination. A few experiments with Melilotus alba 

 have been instituted to test this view, the author believing 

 that loss of water is the cause of these motions and not the 

 effect of light on the chlorophyll. 



Experiment No. 1 was conducted by taking a given lot of 

 potted plants, dividing them into two sets, giving both sets 

 precisely the same conditions of light and heat, but watering 

 one liberally, while the other was allowed to become quite 

 dry. A plant from each lot was photographed from the same 

 direction, and at the same time of day. PI. X, Fig. 1 shows 

 the one with insufficient water, while PI. X, Fig. 2 shows 

 the one plentifully watered. Although the day was not a hot 

 one, yet the plant in dry soil has its leaves very well pointed 

 toward the sun, while the plant in wet soil has only here and 

 there a leaf, some of the younger ones, turned toward the 

 sun. The results here obtained are given on a very large 

 scale in a gravel pit behind the laboratory. Hundreds of 

 plants are growing both on the high, dry ridges, and also in 

 the much moister excavations below. The plants on the 

 ridges are very active in taking the hot sun position, while at 



