4 MINUTES OF 



On the Structure of Vegetables. 



Jially in the moist earth on the roots of trees, and also into the air on their trunks; 

 and thus mix the aqueous fluids, which they thus imbibe, with the saccharine 

 and mucilaginous materials deposited previously in the alburnum of these roots 

 and trunks. During the heat of a summer's day, the juices are highly rarefied, in 

 those vessels; and, by the attractive power inherent in all capillary tubes, by the 

 mutual attraction between the constituent parts ot vegetables, by the irritability 

 in those vesselsj and by the perspiration constantly going on from the superior 

 parts of those vessels, the juices are raised from the roots and other parts of th-; 

 plant to the extremity of their branches, for its developement and increase, and 

 to serve for the various secretions and depositions; by means of which every part 

 i5 formed by the continued impulse of the formative nisus, or power of vegetation. 

 And it is wonderful that these vessels, which are found in the alburnum, and con- 

 sist of a spiral line, whether they may properly be called absorbent or umbilical 

 vessels, or consist of both, should ever have been supposed to be air-vessels. But 

 there is another insuperable objection to this idea of their use, which is, that these 

 vessels equally exist in the roots of plants as in their trunks; and probably termi- 

 nate, externally only in the roots; and, as they are there not exposed to the 

 atmosphere, they cannot serve the purpose of respiration; air nevertheless in its 

 combined state, or even as dissolved in water, may be absorbed by these \essels; 

 but as the solar heat declines, the juices condense, and fall down in the manner of 

 the spirits of a thermometer. What seems to strengthen the opinion of the sap 

 heing thus moved, is, that nature has made no apparent provision whereby the sap 

 might be prevented from descending, in the very same vessels through which it 

 ascends. In the vessels of animals, Avhose office it is to return a fluid, there is an 

 apparatus called valves, which efiectually prevent the contained fluid from going 

 back. These valves are entirely wanting in the vessels of vegetables. The bark 

 of trees annually becomes alburnum or sap-wood; and that sap-wood gradually 

 loses all vegetable life, and becomes heart-wood, and forms a ligneous ring. 



5. Th e Wood, or Lignum, is the compact fibrous substance, disposed into 



