ON LEAVES. 53 



A great variety of experiments have been made in or- 

 der to ascertain the degree of velocity with which the 

 sap rises ; but as the rapidity of its ascension depends in 

 a great measure upon the means which the plant has of 

 parting with it by exhalation, we cannot well follow its 

 progress without having previously made acquaintance 

 with the excretory organs of plants the leaves, whose 

 office it is to exhale that portion of the sap which is su- 

 perfluous. 



Caroline. The whole of the sap then is not required 

 for the nourishment of the plant ? 



Mrs. B. That nourishment is a more complicated op- 

 eration than you are aware of: all the water which en- 

 ters into the plant is not retained by it ; part of it passes 

 through the leaves into the atmosphere, and the atmos- 

 phere, inTts turn, contributes to the nourishment of the 

 plant. But we must not anticipate ; and, at our next in- 

 terview, we will examine the structure and agency of the 

 leaves of plants. 



CONVERSATION IV. 



ON LEAVES. 



Mrs. B. There is nothing more beautiful in the veget- 

 able creation than the gradual formation and development 

 of a leaf. It consists of the flattened expansion of the 

 fibres of the stem from which it shoots, connected togeth- 

 er by a layer of cellular tissue called the pabulum, and 

 the whole is covered by a delicate coating of cuticle, 

 which is almost always of a green color. A plant may, 

 indeed, be considered as a continued series of these fi- 

 bres, sometimes closely bound up in the form of stems, at 

 others spread out into that of leaves. 



Caroline. Yet surely, Mrs B., there are many parts of 

 a plant which can neither be referred to leaves nor stems ? 

 The blossom, the fruit, and such occasional appendages 



266. Why cannot it be told with what degree of rapidity the sap rises'? 

 267. Is all the water which enters the plant retained for its nourishment 1 ? 

 268. What does Mrs. B. say of the gradual formation and development 

 of a leaf? 269. Of what does it consist! 270. What may a plant 

 be considered ! 271. What parts of a plant does Caroline supnos^ - 

 not be referred to the stem or leaf? 

 5* 



