SECT. II. TUBES. 353 



which is that of the juices contained in them ; 

 being white, as in Euphorbia ; or yellow, as in 

 Celandine ; or scarlet, as in Piscidia erythrina. 

 In the plant they are united in bundles, but are 

 detachable from one another by means of being 

 steeped for a few days 'in spirit of turpentine, when 

 they become altogether colourless and transparent, 

 because the resinous matter which they contained 

 has been dissolved. Senebier says they retain their 

 cylindrical form even in their detached state ; and 

 if so, the membrane of which they are composed 

 must be very strong.* 



* ARTICLE 2. Porous Tubes. The porous tubes Descrip- 

 resemble the simple tubes in their general aspect ; 

 but differ from them in being pierced with small 

 holes or pores, which are often distributed in re- 

 gular and parallel rows. They are found in most 

 abundance in woody plants, and particularly in 

 wood that is firm and compact, like that of the 

 Oak ; but they do not, like the simple tubes, seem 

 destined to contain any oily or resinous juice. 



ARTICLE 3. Spiral Tubes. The spiral tubes Descrip- 

 (Pl. VIII. Fig. 17-) are fine, transparent, and 

 thread-like substances, occasionally interspersed 

 with the other tubes of the plant, but distin- 

 guished from them by being twisted from right to 

 left, or from left to right, in the form of a cork- 

 screw. They occur in most abundance in herba- 

 ceous plants, particularly in acquatics ; but thev 

 * Phys. Veg. vol. i. p. 9?. 



VOL. I. 2 A 



tion. 



