76 THE LEAF 



" Sleep of Plants" to all such movements from the evident 

 suggestion of rest. A definite advantage is gained by 

 the nocturnal position. The surfaces of the blades being 

 vertical, or nearly so, and the several leaflets brought to- 

 gether in a cluster (in the case of compound leaves), there 

 is less likelihood that the leaves will be chilled or, in cool 

 climates, frost-bitten. 



118. The "Sensitive Plant." The most striking exhibition of 

 leaf movements after stimulation is perhaps given by the house plant, 

 known from its peculiar behavior as the Sensitive Plant (Mimosa 

 pudicd). The merest touch on one of the leaflets causes the suc- 

 cessive closing together of all the neighboring leaflets, or perhaps all 

 parts of the entire leaf. If the shock is slightly increased, the effect 

 may not only traverse the entire leaf and cause it to droop on the 

 stem, but be transmitted to the other leaves as well. 1 



119. Leaves without blades. fn a few cases the blade of the leaf 

 is quite lacking, while its place is supplied by the enlarged and flat- 

 tened petiole. Certain Acacias of Australia normally have no other 



foliage. In the seedling, 

 however, leaves appear 

 bearing blades. As the 

 seedling grows older, the 

 petioles of these bladed 

 leaves are seen to be flat- 

 tened. Finally the blades 

 fail altogether, on leaves 

 produced at a little later 

 60. Terminal portion of the shoot of a seed- . stage, only phyllodes (phyl- 



ling Acacia: 1, the last of the seedling i odia \ ai)Dear i no . rFis- 60^ 



leaves to show true blades; 2 and 3, 



bladeless, flattened petioles, or phyl- Tne flattening is vertical, 



lodes. so that the phyllode (phyl- 



lodium) presents its edges 



to earth and sky. This fact, even in the total absence of blade or 

 blades, would distinguish these formations from normal leaf blades. 



The Blade 



120. Framework and venation. The framework consists of wood, 

 a fibrous and tough material which runs from tne stem through the 



1 The most remarkable effects are produced by applying a flame, as a 

 match flame, to one of the terminal leaflets. The impulse to contraction 

 may often be followed from one leaf to another over the whole plant. 

 Measure the greatest distance to which the stimulus is transmitted. 



