NUTRITION OF VEGETABLES. 149 



Besides the elaboration of sap by the green tissues, other truly respiratory 

 processes are carried on in the plant for the purposes of assimilation : thus, when 

 the seed germinates, it absorbs oxygen and liberates carbonic acid ; a process 

 analogous to the respiratory in animals, and which is continued until the first leaves 

 of the embryo are developed. Similar respiratory processes accompany flowering ; 

 the petals and stamens absorb, by day as well as by night, much oxygen, and emit 

 much carbonic acid ; hence the noxious quality of the air in a room full of plants ; 

 which is greatly increased "by the exhalation of carburetted hydrogen, contributed 

 by the volatile oils to which the perfume of the corolla is due, 



Evaporation is a phenomenon analogous to the pulmonary perspiration of 

 animals, and should be treated of after respiration. Evaporation is one of the 

 most active agents in the ascent of the sap ; it goes on through all the pores on 

 the surface of the green parts, but especially through the stomata ; increasing or 

 diminishing as the surrounding air is drier or moister. 



Leaves possess in a slight degree only the power of absorbing the watery vapour 

 in the air ; and though certain uprooted plants remain fresh for some time, this is 

 due to their losing little by evaporation. So, too, leaves floating with their lower 

 surface on the water do not wither, not because they absorb water, but because their 

 stomata being stopped up, evaporation is arrested. 



Excretions. A plant, after being nourished by the materials of the elaborated 

 sap, rejects by its leaves, glands, bark, and especially by its root, all useless or noxious 

 matters. Thus, to express in a few words the nutritive functions of its life, a 

 vegetable may be said to absorb, breathe, assimilate, perspire, and excrete, 



Direction of the Axis. The stem tends, always to ascend, and the root as 

 uniformly to descend, and even in underground stems the tip of the rhizome always 

 turns upwards. In the Mistleto, a parasite, the seed, fixed to the branch of a tree, 

 germinates on the bark, and always directs its radicle tow r ards the centre of the 

 branch, and its plumule in the opposite direction ; here the tree takes the place of 

 the soil, and the root obeys a centripetal, the stem a centrifugal force. 



Attempts have been made to elude this general law of the direction of axes, 

 by reversing the seeds of young plants, when the root bends round to the earth, and 

 the stem turns upwards. A box of damp earth has been so suspended that seeds 

 could be planted on the lower surface of the earth, the soil being above, air and 

 light below ; still the stems rose into the earth, the roots descended into the air. 



Movements of Leaves and Flowers. Leaves constantly direct their inner surface 

 towards the sky, and their outer towards the earth ; if this direction is reversed by 

 twisting the base of the petiole, the leaf constantly tends to turn round in spite of 

 all obstacles, and if these obstacles be insuperable, it gradually, dies; if the branch 

 be reversed artificially, the petiole twists ; if the reversion is natural, as in weeping 

 trees, the torsion of the petiole is spontaneous, and the inner surface turns towards 

 the sky ; if, finally, a leaf be so suspended that its blade is horizontal and its inner 

 surface is turned downwards, the blade speedily turns round, and resumes its normal 

 position. This instinct of the leaf depends neither on air nor on light, for it is 

 displayed in water and in darkness. But with many species, the state of the 



