SECT. xiv. THE LA TEX. 4 1 7 



vital power, are soon suffocated, and the plant dies. The 

 expiration of oxygen by the leaves is connected with the 

 nourishment of a plant, the inspiration of that gas is 

 connected with its life. 



When the sap is completely organized by respiration, 

 evaporation, and the chemico-vital agency of light, it 

 descends chiefly through the cambium, lying between 

 the liber and the wood. From this layer the sap distri- 

 butes to each organ capable of increase, the requisite 

 nutritious liquids, deposits various organic compounds, 

 and annually renews the cambium. Part of the sap in 

 its descent runs into the wood through the horizontal 

 medullary rays, in the cells of which it deposits starch. 

 The descent of the sap is no doubt due to gravitation. 



The latex is a general name for those white or 

 coloured juices peculiar to some plants. It is sepa- 

 rated in the leaves from the descending sap, which is 

 always colourless, and consists of a clear liquid, thick- 

 ened and coloured by white, yellow, reddish-brown, or 

 green globules floating in it; it does not turn blue 

 under the action of iodine, therefore it does not contain 

 starch. These proper juices differ as much in quality 

 as in colour; some contain fatty matters, others sub- 

 stances of a totally different nature, as caoutchouc ; a 

 few are bland and nutritious, many acrid and poisonous ; 

 some contain alkaloids, others have none. These juices 

 are by no means essential to the life of the plant, for 

 sometimes they are wanting in their most essential 

 parts, and they are found in certain species and not in 

 others most nearly allied. Certain it is, that tropical 

 lactescent plants which do not produce their proper 

 juices when brought to a cold climate, still produce 

 their milk vessels. 



These vessels follow the ramifications of the veins of 

 the leaves in the highest class, and also in some of the 

 monocotyledons. In the stem the milk vessels belong 



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