150 BOTANY FOR BEGINNERS CHAP. 



bright summer days ; a Sunflower during its active life gives out 200 

 times its dry weight of water. 



Transpiration is the giving out of water vapour by a plant. Only 

 those parts of a plant in contact with the atmosphere can transpire. 



The Organs of Transpiration. Stomata and lenticels are the organs 

 by which plants lose the greater portion of the water vapour which 

 they give out. 



Conditions favourable to Transpiration. (i) A certain intensity of 

 light; (2) a dry atmosphere ; (3) a windy day. 



Why Plants Transpire. (i) To get rid of the excess of water taken 

 in by the roots ; (2) to aid the distribution of salts throughout the 

 plant ; (3) to aid absorption of water and salts by the roots. 



Liquid Water given out by Plants. Plants like the Lady's Mantle, 

 Buttercup, and Arum give out liquid water through water pores, 

 stomata, or epidermis. 



Boot Pressure is the power which roots possess of forcing water up 

 the stem. 



How Boot Pressure is Set Up. Root-hairs are very active in spring, 

 and take in large quantities of water until all the cells of the roots are 

 filled. The water from the cells exudes into the vessels of the xylem, 

 and it is then forced up the stem to the leaves. 



Water Travels in Plants up the interior of the vessels of the new 

 wood. 



The Transpiration Current is the current of water which passes up 

 the stem to make good that lost by transpiration. It either passes up 

 the younger and oute rings of the wood (woody plants), or through 

 separate vascular bundles (herbaceous plants). 



Elaborated Sap has been acted on by the chlorophyll and protoplasm 

 in the leaves. It travels in the following ways ( i ) The proteid sub- 

 stances in it pass along the sieve tubes ; (2) the sugars move through 

 the parenchyma cells round the vascular bundles ; (3) a slow movement 

 occurs from cell to cell to make good the loss due to growth. 



QUESTIONS ON CHAPTER XI. 



(1) What part of its food does a green plant obtain by means of 

 roots? How does the root absorb food ? (1897.) 



(2) What do you know about 



(a) The materials found in a fertile soil ? 



(b) The substances found between the particles of a soil? 



(c) The way in which water travels in a soil ? 



(3) Define the term osmosis. Explain the part which osmosis plays 

 in the nutrition of a plant. 



(4) What conditions are necessary for absorption? 



(5) What are the functions of the root? Briefly explain the relation 

 between the structure and functions of a root. (1893.) 



(6) Explain why it is that plants droop on a hot day and recover their 

 freshness in the evening. (1889.) 



(7) What is the "transpiration current"? State by what tissue it 



