84 VASCULAR TEXTURE. [LECT. III. 



they were the vegetable organs of respiration*. 

 They are the largest of the vegetable vessels ; and 

 in many plants their structure is visible to the 

 naked eye. Thus, if a leaf, or a green twig of 

 Dog-wood (Cornus sanguined), or of Elder (Sam- 

 bucus nigra}, or the stem of any of the Lily 

 tribe, or one of the fleshy scales of any bulb, 

 as, for example, that of the Squill, be partially 

 cut, then cautiously broken, and the divided 

 portions carefully drawn asunder, the spiral vessels 

 will be seen appearing like a screw, and their real 

 structure become apparent. They are formed of 

 a thread, turned in a spiral manner from right to 

 left ; as if, to use an illustration of Dr. Thomson, 

 a fine and slender (he should have added flat) 

 wire were wrapped round a small cylinder 

 of wood, so that the successive rings touch 

 each other, and then the cylinder be with- 

 drawn ; the form thus acquired by the wire 

 will represent the spiral tubes ~}~. (Fig. i.) 

 The thread of which they are formed is 

 elastic, opaque, silvery, shining, and flat ; 

 and in several plants, particularly the Ba- 

 nana (Musa Paradisiaca), it is sufficiently 

 strong to suspend the inferior portion of the 

 twig, or the leaf, if it be not very large ; 

 but there is no reason for believing, as 



* Such was the opinion of Grew, Malpighi, Hedwig, and 

 Linnaeus. 



f Thomson's Chemistry, 5th edit. vol. iv. p^ 336. 



