NUTRITION. 81 



47. The crude sap has to be conveyed from the 

 root to the leaves, and the structures through which the 

 transmission takes place vary according to the rank of 

 the plant. In Dicotyledonous plants the upward course 

 of the sap is through the fibrous or woody tissue and 

 ducts of the woody layers, and chiefly of those last 

 formed, the alburnum ; but I think the observations of 

 Gaudichaud, Meyen, and others, prove that it takes 

 place at a certain and limited period through the spiral 

 vessels. In Monocotyledons no doubt a very con- 

 siderable portion is transmitted by the cellular tissue, in 

 which the more limited vascular one is embedded, and 

 by the ducts also perhaps more than by the woody fibre ; 

 but in all cases besides the mere upward course there 

 is likewise a ready lateral motion of the fluid by which 

 all parts of the plant are permeated : this lateral trans- 

 mission takes place by means of the cellular tissue, and 

 the medullary rays when they exist are especial media. 



48. In regard to the cause of the motion of the sap, 

 many have been the theories : Malpighi, Brugmans, 

 etc., believed in an irritability of the smaller vessels, and 

 lately Decandolle has put forward a vital contractility 

 of the cells analogous to the systolic and diastolic mo- 

 tions of the heart ; Knight believed in a hygrometrical 

 property of the medullary rays ; Du Trochet, in the 

 powers of endosmose and exosmose ; Du Petit Thouars 

 in the expansion of the buds at spring attracting a cer- 

 tain quantity of sap from situations near them, and so 

 causing the sap below to ascend and fill its place ; 

 and Hales states, that " no other cause can be discovered 

 than the strong attraction of the capillary sap vessels, as- 

 sisted by the brisk undulations and vibrations caused by 

 the sun's warmth ; but the ascending velocity is prin- 

 cipally accelerated by the plentiful perspiration of the 

 leaves." 



49. In regard to these various theories we may re- 

 mark, that there is as yet no distinct and satisfactory evi- 

 dence of a contractile power in the solids through which 

 the crude sap passes ; that although endosmose and ex- 



