34 VEGETABLE ORGANOGRAPHY. 



to a very distant family. Kieser has remarked, that 

 in the greater part of vegetables the tracheae which 

 are furnished with a simple spire are disposed in bun- 

 dles : on the contrary, we find them sohtary in the 

 Banana, which has the spires multiplied ; whence one 

 might infer, that tliis multiplied spire is formed by the 

 union into one single tube, of filaments usually distinct. 



Malpighi and Reichel say that they have observed 

 contractions in the tracheae, but no subsequent observers 

 have seen them. Mirbel asserts positively that these 

 are optical illusions. The diameter of the tracheas is 

 about the twenty-fourth of a Kne, according to Mirbel ; 

 but according to Kieser, their diameter is very variable 

 in different plants. 



Malpighi says that, " during the %\dnter, the trachcEe 

 are endowed with a vermicular movement, which de- 

 lights the observer." It appears that this anatomist has 

 here attributed to irritability, that which arises simply 

 from hygroscopicity combined with elasticity. A motion 

 in the tracheae, when laid open, may be occasioned 

 either by approaching and removing the ends of a young- 

 shoot broken across, or by exposing them alternately 

 to moisture and aridity. Mirbel asserts that the 

 tracheas of the Butomus umbellatus, once unrolled, never 

 contract again. The tracheas are very visible in most 

 of the yomig shoots of the year, especially in those 

 which can be broken clean off without tearing them, as 

 those of Roses : they are to be found, according to the 

 observations of Mirbel, only around the pith in the 

 old stems of Dicotyledons; for it appears that all that 

 former observers have said of tracheas observable in the 

 wood, must be attributed to the striped vessels : the 

 tracheas appear to be organs essential to the medullary 

 sheath, and they are to be found there, in the form of 

 those which are incapable of being unrolled, even in 



