•FUNCTION OF LEAVES. Ill 



have been compared to the lunfjjs of animals, but they perform 

 much more for the plant, than this comparison would indicate. 

 Thi'V are not only tiie or«j^ans of respiration, hut also of diges- 

 tion and nutrition. They pcM-torrn in every respect for ve- 

 getables what are perlorined hy the lungs and stomach, and 

 the whole digestive apparatus, in animals. They receive 

 the crude sap tVom the roots through the stem and elaborate 

 it bv exposiiirj it to tl»e action of the atmosphere, throwing 

 otr the suportluous moisture, decomposing water and carbonic 

 acid, sending iK)\vn the deleterious substances to be voided by 

 the roots. They send inmiediately downwards the materials 

 of the albumen and liber, and nourish with this elaborated 

 food the contiguous parts. 



Tiiat the nutrition of a plant depends upon its leaves is 

 abundantly proved by depriving a plant of these organs through 

 a season, and it withers and dies. It does not die immmedi- 

 atelv, since it possrs es the power of putting forth new leaves 

 which soon come into action ami supply imperfectly the pla- 

 ces of those removed ; but if it is deprived of its leaves through 

 the season, its power of putting them forth becomes cAhausted 

 and all functions cease. 



The presence of cotyledons also shows the necessity of leaves 

 to prepare food for the embryo. If the cotyledons be removed 

 the seed seldom germinates, and if it does, it is in a sickly 

 state. The structiwe of the leaf shows its ada[)tati()n to the 

 purposes of respiration. We now proceed to describe some 

 other of its functions. 



136. By w hat we have cnlled crude saj), we do not mean that 

 it is not changed at all in its ascent through the root and stem, 

 but that it is unfit for assimilation, until it has passed through 

 the leaves. Prof. Emmons publislu;d an article in the Amer- 

 ican Journal of Science, for 1834. vol. 26. p. 99, in which he ar- 

 gues against a double circulation in vegetables, and the effect of 

 the leaves in ela!)orating the sap. From the well known tal- 

 ents of the Prof, and our own estimation of his exalted abili- 

 ties on any subject to which he turns his attention, we have 

 taken uncommf)n pains to settle this question in our own 

 mind, and we iiave come, by various ex|)eriments, to the con- 

 clusion, that all the functions we have attributed to the leaves 



tire certainly performed by them. As the Professor throws out 

 his ideas in the form of hints, we should be gratified to know 

 whether future observations confirmed his opinions. We 

 have seen nothing from his jien on the subject since the arti- 

 cle referred to above. 



137. It would be an important point to determine the real state 



