86 VASCULAR TEXTURE. [LECT. III. 



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 off,, and enter the middle rib of 

 leaves, to be distributed through them under their 

 upper surface. They have been detected, also, in 

 the calyx, or flower-cup, and other parts of the 

 flower ; and Gsertner asserts that they are evident 

 even in the seed-lobes. The spiral vessels, in their 

 course, proceed always in straight lines, without 

 any deviation; whereas all the other vegetable 

 vessels often take a curved direction. It is into 

 these vessels that coloured injections most easily 

 enter ; and when an annual twig of the Fig is thus 

 injected, they are seen in a transverse section of it, 

 like red dots around the pith, placed within an 

 external circle of the vessels, which contain the 

 succus proprius, or milky juice of the plant. 



It cannot be affirmed that the varieties of form, 

 which we have pointed out in the vegetable 

 vessels, is of the same importance as the differ- 

 ence which exists in the structure of the arteries 

 and veins of animals. There are, indeed, some 

 plants in which three of the modifications of struc- 

 ture, according to Mirbel's observation, are found 

 in the same tube. In the Butomus unibellatus, 



