EXHALATION. RESPIRATION. LEAVES. 29 



mounts in the grape vine, it was found to be sometimes so great 

 as to sustain the weight of a column of water, over forty feet in 

 height. 



10. The circumstances that have most influence upon the as- 

 cent of the sap are heat and light. 



OF EXHALATION AND RESPIRATION. 



11. To render it fit for the purpose of nutrition, the ascending 

 sap undergoes, in the interior of the plant, considerable changes ; 

 these changes are the result of two important phenomena ; name- 

 ly, exhalation and respiration. 



12. The leaves are the chief seat of these two functions, and 

 must be regarded as their special organs. We will now study 

 their structure. 



OF LEAVES. 



13. The leaves of vascular plants are the lateral appendages 

 of the stem, formed of more or less distinct fibres and cellular 

 tissue, enclosing, in its interior, a great deal of green colouring 

 matter. 



14. The fibres of the leaf are the continuation of those of the 

 stem, but ordinarily they contain more trachece ; in general, 

 they form at first a cylindrical fasciculus (bundle), caniculated 

 (that is, hollowed in a gutter on the opposite side), or laterally 

 compressed, which is named petiole, or leaf-stalk ; then they 

 expand and join again to form the flat part called the blade or 

 limb of the leaf. When the fibres separate immediately on 

 springing from the stem, the leaf has no pedicle or petiole, and 

 is then said to be sessile (from the Latin, sedeo, I sit). The 

 petiole of dicotyle'donous* plants is separated from the stem 

 by an articulation or joint, that is, a line at which its tissue offers 

 but little resistance, the cells and vessels of which it is composed 

 being placed end to end, instead of being mingled as usual ; it is 

 on account of this arrangement that the leaves fall when they 

 fade, while those of which the limb or blade arises directly from 

 the stem are destroyed only little by little, and remain adherent 

 at their base. The first are called caducous, or articulate leaves, 



* DICOTYLE'DONOUS. (From the Greek, dis y double, and kotuledon, a seed 

 leaf.) Having a double seed-leaf or seed-lobe. 



10. What circumstances most influence the ascent of the sap? 



11. Does the ascending sap undergo any change in the interior of 

 plants ? 



12. In what part of plants do exhalation and respiration take place ? 



13. What are leaves ? 



14. How are leaves formed ? What is the petiole ? What is the limb of 

 a leaf? When is a leaf said to be sessile ? What are caducous or articu. 

 late leaves ? What are persistent leaves ? 



3* 



