NUTRITION. 59 



at its entrance into the plant, if its course has 

 been rapid through the older wood,* where the 

 particles are slightly soluble ; that on the con- 

 trary which has traversed those younger por- 

 tions in which there is much cellular tissue filled 

 with nutritive particles, slackens its course, 

 mixes with and dissolves them, and arrives at 

 the higher parts of the plant loaded with nou- 

 rishment. The cells appear to be the true 

 organs of nutrition, in which the decomposition 

 and assimilation of the juices takes place. In 

 each cell ligneous matter is deposited which 

 coats its walls, and the inequalities of this depo- 

 sit in many cases appear to have given rise to 

 the idea that the cells were perforated the 

 thinner portions being so r transparent, that 

 under the microscope they have the appearance 

 of pores. It is evident from the above detail 

 that there is no circulation in plants strictly 

 similar to that of animals, but that there is an 

 alternate ascent and descent of the sap. 



45. It will be gathered from the account of 

 the course of vegetable nutrition just given that 



* It has been proved by colouring the water with co- 

 chineal, that the ascent of the sap certainly takes place 

 through the ligneous system, though the particular chan- 

 nels may be doubtful. 



