104 BOTANY. [CHAP. m. 



protoplasm. The closing and opening of leaves is well seen 

 in the wood sorrel (Oxalis acetosella), many of the clovers 

 (Trifolium) , etc., and more especially in what is popu- 

 larly known as the " sensitive plant " (Mimosa pudica), 

 where this faculty has become so thoroughly perfected 

 as to be not only influenced by the amount of light, 

 which usually causes the closed or " sleeping " condition 

 during darkness, and the " waking " or expanded con- 

 dition during the day, but to respond at once to the 

 slightest mechanical irritation, and in this case the trans- 

 mission of local irritation from one part of the plant to 

 another, equivalent to the transmission by nerve-power 

 in the animal world, causes every leaf on the tree to 

 close within a minute or two after the slightest touch at 

 any one point, even the irritation caused by touching a 

 leaf with, say, the point of a pencil. 



The amount of work done by a given area of leaf 

 surface depends on the amount of light received by 

 every portion of that surface ; consequently the arrange- 

 ment of leaves from this point of view is of great impor- 

 tance. In many Monocotyledons, as the grasses, sedges, 

 and most bulbous plants, most or all the leaves spring 

 from one point near the ground ; this is also the case 

 with some Dicotyledons, as the dandelion, dog-daisy, 

 primrose, etc. Such an arrangement of leaves in the 

 form of a rosette is a primitive and comparatively im- 

 perfect method, inasmuch as some of the lower leaves, 

 after costing energy and material to make, are of little 

 service to the plant, being overshadowed by the upper 

 leaves ; this weak point was remedied to some extent 

 by the development of a tall stem that elevated the 

 leaves and exposed them to more favourable conditions 



