SECT. II. TUBES. 355 



Thomson's. Take a small cylinder of wood, and 

 wrap round it a piece of fine and slender wire, so 

 as that the successive rings may touch one another, 

 and then pull out the cylinder. The wire as it 

 now stands will represent the spiral tubes as they 

 exist in the plant. And if it is stretched by pull- 

 ing out the two extremities, it will represent them 

 in their uncoiled state also.* 



But although the spiral tubes are to be met with 

 in almost all plants, they are not yet to be met 

 with in all the different organs of the plant ; or at 

 least there are organs in which they occur but 

 rarely, or in very small numbers. 



They do not seem to occur often in the root, or Not easily 

 at least they are not easily detected in it. Grew 

 and Malpighi do indeed represent them as occur- 

 ing often in the root, the former referring for ex- 

 amples -f- to the roots of plants in general, and the 

 latter^ to those of the Asparagus, Poplar, Con- 

 volvulus, Elm-tree and Reed ; all of which I have 

 examined with great care, without being able to 

 discover any spiral tubes. Senebier says he found 

 them in the root of the Balsams and Thorn-apple ; 

 in examining which I was equally unsuccessful as 

 in examining the former. I cannot, however, doubt 

 the accuracy of the /observations of the above phy- 



* Thomson's Chemistry, vol. iv. p. 405. 

 f Anatomy of Roots, chap. iv 

 + De Radice Plantarum. Opera Ornnia, 

 Senebier, Phys. Veg. vol. i. p. ICty, 

 2 A 2 



