622 The Outline of Science 



tunities of clasping proper supports. The tendril responds only 

 to the touch that helps it. It is to the same stimulus that the 

 sundew tentacles react when they fold on a struggling fly, though 

 the movement is intensified by the insect's chemical action. 



Light and other Influences 



The seedling, fresh from the protection of the seed-coats, 

 young and delicate, is at once affected by all the influences of the 

 world into which it has come. Gravity is not the only directive 

 agency. The root grows against some shard, is slightly wounded, 

 and turns away from the source of injury. It rubs gently against 

 grains of soil, and the contact causes it to move gently to and 

 fro; it feels its way through the crevices of the soil. It comes to a 

 dry region and bends to the moister side. But the shoot, subject 

 to all these stimuli in the soil, finds yet another director in light 

 when it reaches the air. 



Everyone knows the window plant, with the shoots bent to 

 the light, the leaves twisted round and spread flat to catch the 

 rays. Light interferes with the action of gravity on the stem, 

 and is the primary cause of leaf movements. Its action is compli- 

 cated by the fact that, unlike gravity, it is constantly changing 

 its intensity and direction. Few plants can react so quickly as 

 to follow the sun; moreover, as growth ceases, the power of move- 

 ment ceases, except in the case of those leaves which have jointed 

 stalks. So it comes about that most leaves, as they mature, adopt 

 a fixed position in relation to light. 



7 

 Sensitive Plants 



The Sensitive Plant is a vigorous shrub, usually reaching a 

 height of some feet in our hot-houses. Each graceful leaf has a 

 stalk jointed to the stem, from the tip of which spring four sec- 

 ondary stalks, each with a double row of leaflets; the foliage 

 stands out from the stem, the leaflets are expanded. If the plant 



