306 JOUKNAL OF THE EOYAL HORTICULTURAL SOCIETY. 



expand and its parts to grow, but before they have escaped, and while the 

 relative positions are still maintained. 



We have seen that the object in having the young leaves con- 

 duplicate, and in placing them in a vertical plane, is to reduce the 

 injurious effects of the loss of heat by radiation to a minimum ; so 

 that the upper surfaces especially should not be chilled, and thereby 

 injured or even killed. 



There are many plants of which the foliage is said to sleep at nights ; 

 and the object is precisely the same. The phenomenon is particularly 

 common in the Pea family (Leguiniiiosce) ; and it is interesting to see 

 how different plants having similarly formed leaves yet sleep in very 

 different ways. As good examples to illustrate this let us take the Clover,. 

 Medick, Melilot, and Wood-sorrel, as all of these have "trifoliate" 

 leaves — i.e. the compound leaf consists of three leaflets on one main 

 petiole. It is worth while contrasting the form of the young leaf with 

 that of the adult leaf when asleep. The Clover leaf and that of Wood- 

 sorrel are, when undeveloped, precisely alike. The petiole is curved so 

 as to place the blades in a vertical position, while all the three leaflets 

 are tightly pressed together with conduplicate blades, so that a cross 

 section shows six edges placed side by side. 



When, however, a full-grown Clover leaf goes to sleep from having 

 had the three leaflets spread out horizontally by day ; first, the two lower 

 leaflets rotate through 90°, so as to stand vertically ; then they move 

 horizontally till they meet in front, with their upper surfaces in contact. 

 Finally, the terminal leaflet rises up, passes through a semicircle, or 

 180°, and comes down like a sloping roof over the upturned edges of the 

 other leaflets. Thus it remains till the following morning. 



Comparing this with the Wood-sorrel, though its leaflets were com- 

 pacted exactly in the same way as those of the Clover when very yoimg, 

 yet when they sleep they simply drop vertically and slightly bend along 

 the midrib, so that the three leaflets can fit in closely, their midribs 

 lying along the petiole. 



Medicago has also a "trifoliate" leaf, and behaves precisely like a 

 Clover leaf, but the Melilot does not. The lower leaflets move as in the 

 Clover, but the terminal leaflet behaves differently. It places itself 

 parallel to the other two in a vertical plane, and then moves to one side 

 and places its upper surface in contact with that of the leaflet w^hich it 

 approaches. We thus find at least three different methods among four 

 plants. 



Lupines are interesting as also furnishing three methods in as many 

 species. Thus Lupinus lyilosus drops its leaflets when asleep, as shown 

 in fig. 162 ; but L. Hartivegii raises them like an inverted shuttlecock ; 

 while L. puhescens differs from both, in that while the shorter leaflets 

 fall the larger ones rise ; the result being that the whole of the leaflets 

 are in a vertical plane. The leaf has thus changed from being, so to 

 say, a horizontal star to a vertical one. 



