220 STRUCTURE AND VITAL PROCESSES OF PLANTS 



that tliere is no oxygen in the bottle. The latter must have 

 been absorbed by the growing seeds which gave off carbonic acid 

 instead. 



(b) Necessity of Respiration. — Living plants have to do work 

 of various descriptions: they lift their parts against gravitation, 

 they force their roots through hard soil and often break up even 

 masonry work, they grow and form new cells, they move tendrils 

 and press winding stems against their supports. Work always 

 needs a supply of energy, just as in animals or in an engine. 

 The source of power underlying the work of animals is derived 

 from their food, the use of which is connected with their respira- 

 tion by which carbon is burnt and heat is produced. So it is 

 with plants also, though their work may be slow and easy, and 

 hence the need of such energy less than in animals. 



In the green parts the action of respiration at day cannot be 

 shown as they assimilate under the influence of sunlight more 

 vigorously than they respire. They rather appear to exhale 

 oxygen. We have, therefore, to clistinr/uish strictly beticeen As- 

 similation and Respiration. While in the process of assimilation 

 green plants alone, and only in the light, decompose carbonic acid 

 and give off oxygen, ^\ i)la)it-organs, luithout exception, both by day 

 and by night take up oxygen and give off' carbonic acid. 



When the action of respiration is vigorous, e. y.j in young leaves, 

 or in wounded ones, it is indicated by a red hue (see Mango, Cinnamon), 

 due to a red colouring matter, called erythrophyll*. This pigment is 

 also present in flowers and fruits coloured red or blue, according as 

 it is dissolved in an acid or alkaline cell-sap. It appears, too, in old 

 leaves, painting the foliage of northern forests with that charming 

 red before they drop their leaves. 



If plants or parts of them are deprived of oxygen and the 

 action of respiration is thus checked, they are hindered in their 

 growth, become sickly, or perish. This can often be noticed in 

 pot-plants or fruit-trees which are planted too deeply and covered 

 with too much eartli. Tlicir roots cannot get the re(juired air for 

 respiration. Conversely, the loosening of the ui)pcr crust of soil 

 is advantageous to crops to allow the air free access to the roots. 



♦From Greok erylhros, red, and phj/lloii. a leaf. 



