232 THE SOLID COMPONENTS OF PLANTS. 



wire will represent the spiral tubes. The thread of 

 which they are formed is elastic, opaque, silvery, shin- 

 ing, and flat ; and in several plants, particularly the 

 Banana, the Hcemanthus, and several species of Ama- 

 ryllis, is sufficiently strong to suspend the inferior por- 

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

 there is no reason for believing, as Willdenow and 

 others have asserted, that it is hollow, and forms a 

 real vessel thus twisted in a spiral manner ; or, that- 

 the larger hollow tube is an air-vessel, while the spi- 

 rally twisted thread is a vessel carrying fluid. For, 

 if we consider the smallness of the larger tube, and 

 the flattened state of the thread of which it is formed, 

 the impossibility of any fluid entering the smaller one. 

 if it really existed as a vessel, may be easily conceiv- 

 ed. According to Hedwig's observations, made with 

 a microscope which magnified 290 times, he found 

 that the apparant diameter of these air-vessels, as he 

 supposes them to be, is one tenth of an inch ; their 

 real diameter, must therefore, be the 290th part of the 

 tenth of an inch, or the 2,900th, part of an inch. 

 What then, I would ask, must the diameter of the 

 supposed spiral vessel be, and what fluid could be 

 conducted through it ? The thread is sometimes 

 double ; and Mirbel asserts, that it is furnished with 

 a glandular border. 



These vessels are found in great numbers in mono- 

 cotyledonous plants, as in the centre of the ligneous 

 threads, which exist in the stems of Grasses, and in 

 Palms. They are numerous also in most herbaceous 

 plants ; and particularly in aquatics of a lax texture. 

 They are seldom detected in the root, and never in 

 the bark ; but are situated round the medulla of the 

 young shoots of trees and shrubs ; whence bundles of 

 them are given oft, and enter the middle rib of leaves, 

 to be distributed through them under their upper siu> 



