338 PRINCIPLES OF BOTANY, ETC. 
more or lesscompresses them. We see them, there: 
fore, of an elliptical, spherical, compressed, or COs 
nical figure. ‘They are largest where the stem ends 
and the root begins, but decrease in thickness to- 
wards the superior part of the stem, and towards 
the ends of the root. In general we can see the 
vessels much more distinctly in young plants, where 
they are largest, than in old ones, which are more 
lieneous. | 
Some Hots have thought that these vessels are 
formed out of the cellular texture. But it is not 
very probable that they owe their origin to the cel- 
lular membranes, as these are by far too irregular, 
and as they are found already formed in. the corcle 
of the seed. 
Ve shall soon find that they harden along with 
the air vessels and the wood, and that they consti- 
tute the ligneous fibre, which is to be well distin. 
guished from the muscular fibre. With this the 
ligneous fibre, being an indurated vessel, has not 
the least resemblance ; besides which no other part 
occurs resembling the animal fibre. But as Mr Van 
Marum’s experiments have proved, that the vital 
power causes them to contract, by which the sap is 
pushed forward, it may be asked, whether these 
ligneous fibres themselves are not composed of thin 
muscular fibres, ‘or at least of a sort of aponeurotic 
membrane? But this point will surely never be de- 
cided, as the vessels are so very minute themselves 
that we must rest satisfied, even with the aid of a 
microscope, merely to. ascertain their existence. 
Tt certainly would be very difficult to produce any 
thing 
