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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



are packed up within the bud. As the buds open and the leaves 

 begin to grow, they endeavour to secure two objects, viz. to 

 protect their upper surfaces and to grow in as perpendicular 

 direction as possible. The advantage derived from this position 

 is to reduce or counteract the ill effects of a chill by radiation, 

 which is much greater from a horizontal surface than a vertical 

 one. The leaves and leaflets are generally conduplicate, i.e. 

 folded together, with their upper surfaces inwards. This results 

 from the lower surface growing faster than the upper in the 

 earliest stages of development ; so that the two upper halves of 

 a leaf are pressed together. The vertical position is acquired 

 by one side of the petiole growing faster than the other ; so 

 that half-developed leaves are either erect or dependent. Sub- 

 sequently the rates of growth are reversed, so that the leaf 

 assumes its final and horizontal position with the upper surfaces 

 most exposed, more or less at right angles, to the light. 



Sleep. — The positions assumed by growth in young leaves 

 are analogous to those which are sensitive to varying degrees of 

 intensity in the solar radiations. In Clover, e.g., the three leaflets 

 are conduplicate, and they are all compressed together in the 

 immature stage and hang down vertically ; but when mature 

 and " going to sleep " at sunset the two lateral leaflets rotate 

 till they are vertical. They then approach each other and press 

 their upper surfaces together, while the terminal leaflet passes 

 through half a circle and comes down over their upturned edges 

 like a roof. 



Thaumotiiopism. — If the soil be warmer than the air just 

 above it, plants will often grow with their stems and branches 

 prostrate upon the ground, and so become creeping plants. 

 Thus the common Mallow is usually erect when surrounded by 

 other plants ; but in the barren, rocky "Soil of Malta it is quite 

 prostrate, creeping over the ground to considerable distances by 

 road-sides. Similarly, plants with a rosette of leaves as Plan- 

 tains and Daisies on lawns, Blue-bells in early spring, &c, have 

 their foliage flat upon the ground from a similar cause, " turning 

 heatwards." The same peculiarity is particularly common in 

 dry Alpine regions.* 



Movements of Flokal Ohgans. — These are very numerous, 

 and may occur in the flower-stalks or among the floral organs, 

 * See Origin of Plant Structures, p. 104. 



