THE VESSELS. 41 



Section VI. 

 Of Reticulated Vessels. 



This form of vessel (PI. 2, fig. 7 o,) is extremely rare iii 

 nature, and has been studied the least of all. Kieser has 

 only discovered them in the Balsam and the Nasturtium 

 {Tropceolum) : he suspects their existence in other plants 

 of a loose texture. According to this observer, these ves- 

 sels are owing to the spiral or annular fibres which compose 

 the trachese, or striped vessels, anastomosing unequally 

 together, and leaving between them open spaces or 

 oblong holes. They never attain, according to him, the 

 size of dotted vessels, and are often ramified ; they are 

 more frequent in the root than in the stem. 



Section VII. 

 General Considerations on the Structure of Vessels. 



I have described, in the preceding sections, the 

 usual forms which the vessels of plants present 

 under the microscope ; and I have intentionally avoided 

 confusing these descriptions with any hypothetical or 

 even theoretical ideas. It remains for us now to examine 

 what are the modifications of which these forms are 

 susceptible ; and thence to deduce, if possible, the 

 relation of these different vessels with one another, and 

 their true nature. 



Hedwig was the first to treat of these delicate ques- 

 tions in his Programme upon Vegetable Fibre : he 



