THE STEM 177 



appears when a vegetable organ is wounded, that is, 

 just when it becomes exposed to abnormal evaporation, 

 and thus puts an end to this unhealthy condition. 

 Thus, for instance, we have only to wound any vege- 

 table organ and thereby lay it bare and leave its inner 

 tissues unprotected, and in a short time we shall see 

 the skin of cork tissue forming over the wound. 



Thus the root drives water into the stem, the stem 

 carries it along to the leaves, the leaves evaporate it into 

 the air. It is only in the conjoint and uniform fulfil- 

 ment of all these functions that the activity of the plant 

 will be completely normal. The balance is upset when 

 the plant evaporates more than it absorbs — then it 

 withers ; the balance is also upset if the plant has no 

 time to evaporate all the water it absorbs — then it 

 begins to exude it in the form of drops, such as we notice 

 on the blades of grass on warm damp evenings when, 

 owing to the saturation of the atmosphere with water 

 vapour, evaporation from the leaves has almost ceased. 



We pass now to the investigation of another movement 

 of the nutrient substances, tending not towards the leaf, 

 but from the leaf towards all parts of the plant, including 

 the root. The fact that there must be such a movement 

 is evident a priori, because the organic matter out of 

 which all the parts of the plant are built up is formed 

 in the leaf ; the fact that it actually exists is clearly 

 proved by the following simple experiment. Let us 

 cut off a willow branch and place it in water. After a 

 few days or weeks a kind of excrescence appears round 

 the lower cut end of the branch, and from this excrescence 

 there spring little roots. Evidently these rootlets must 

 have been formed at the expense of matter obtained 

 from the leaf, or else of matter which was already on 

 the way from it, i.e. in the stem. Let us try to show the 

 way by which it has come down to the newly formed 



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