28 ASCENDING SAP. 



stem of a plant is cut across, its internal tissue being thus laid 

 hare, also pumps up water in which it may be placed ; but in the 

 ordinary state of a plant, these cases are exceptions, and the ab- 

 sorption of liquids is carried on in the most active manner by the 

 spongioles. 



5. It has been remarked that water, rendered thick and viscid 

 by the presence of foreign substances, was absorbed very slowly 

 and willi difficulty, but when its fluidity is not diminished by 

 matters that it holds in solution, it penetrates vegetables just 

 as if it M^ere pure. Now, the water which reaches the roots 

 of plants always holds in solution a greater or less quantity 

 of air, earthy salts, and organic matter ; and consequently it 

 introduces these substances into the interior of the plant, which 

 is either benefited or injured according as they are proper for 

 its nutrition, or as they exert an injurious influence upon its 

 organs. 



6. The liquids thus absorbed by the roots constitute the as- 

 cending sap, which rises through the stem to reach the leaves. 



7. The ascent of the sap is always effected through the ligneous 

 body ; and it is remarked that it takes place more actively through 

 the albur'num than through the perfect wood. 



8. It is not known with certainty by what way the absorbed 

 liquids rise up in this manner; many botanists think that it is 

 only by the intercellular passages ; others believe that it is by 

 the vessels; and in fact, if we place the roots of a plant in 

 coloured water, we are not long in perceiving that the vessels 

 of the stem assume the same colour, which seems to indicate that 

 it is through these tubes that the liquids mount up towards the 

 leaves. Nevertheless, under ordinary circumstances, we find 

 these vessels empty, or at least filled with air, and it would seem 

 that it is chiefly through their interior that the air, absorbed by 

 the roots, ris^s in the stem of the plant. 



9. The rapidity and force with which the ascent of sap takes- 

 place, are sometimes extremely great. In the experiments made 

 upon this subject, it has been shown that a branch of an apple tree 

 cut across and surmounted by a tube, raised water contained in 

 the laiter several feet in the space of some hours ; and what 

 are called vine tears, is nothing but the ascending sap, which 

 escapes in abundance when the plant is trimmed. In other 

 experiments made to ascertain the force with which the sap 



5. How are earthy salts introduced into the substance of living plants ? 



6. What constitutes the ascending sap ? 



7. Through what part of the plant does the sap ascend ? 



8. What is the manner of the ascent of the sap ? 



9. What is the force and rapidity of the ascent of the sap? 



