VOL. LXVI.] PHILOSOPHICAL TRANSACTIONS, ^1^ 



of the gorgonia and that of trees ; and that is, in the connection between the 

 side branches and stem of the one, and the side branches and stem of tlie 

 other. The side branches of vegetables proceed from the pith ; of course, when 

 a stem and side branch is divided lengtliwise, the pitli is seen continued through 

 the main stem into the branch ; see fig. 10, where a is the natural size of a 

 small twig of the lime tree, and b the same magnified. It must be observed, 

 that in some trees the channel or continuation of the pith, which leads from 

 the stem to the side branch, is very much contracted, and the communication 

 very narrow ; in which case it will be necessary to make cross sections, which 

 will soon discover the course of the pith from one to another. M. Du Hamel, 

 an author of the first reputation, has clearly demonstrated this in his Physique 

 des Arbres, vol. 2, p. UQ, tab. 2, f. Ql. Now in the gorgonia, the support, 

 or what is called the woody part, is indeed furnished with a kind of a pith 

 or medulla : but when we cut the stem or branch through the middle 

 lengthwise, we find no passage whatever between the pith of the stem and that 

 of the branch, each being surrounded with a hard covering of its own, which 

 has no perforation, nor admits of any communication. Every branch of a gor- 

 gonia therefore has its own pith or medulla peculiar to itself, which is never 

 found passing into that of another, see fig. 1 1 , a, the natural size, b magnified. 

 Again, in trees, the pith is largest in young shoots, and disappears in old stems : 

 in the gorgonia the medulla is of the same diameter in the old stems as in the 

 young branches. In the longitudinal sections of fresh shoots of trees, the pith 

 in the microscope looks like a number of jointed tubes united together;, and in 

 the cross sections, it appears like so many circles. In dried specimens the 

 tubular appearance in the longitudinal sections is more irregular ; they look 

 rather like longitudinal ranges of little transparent blebs, and the cross sections 

 appear like circles intersecting each other in the margin ; but there are many 

 varieties of figures in the pith of different vegetables ; what is mentioned here, 

 is the common appearance of pith in most plants. When we cut a dry branch and 

 stem of a gorgonia through the middle lengthwise, the pith appears divided into 

 many little transverse membranes, like small white diaphragms, separated from 

 each other about the distance of their own diameter. But these cross mem- 

 branes are found to be more numerous in such as have been preserved directly 

 from the sea in spirits ; and when they are examined in the microscope, they 

 appear to be of the nature and substance with the laminae that compose the horny 

 tube that surround them.* 



• While comparing the longitudinal sections of the young branches of trees with those of the gor- 

 gonia, I was surprized to find such a similitude between the pith of a branch of a walnut tree, of a 

 year's growth, and that of the gorgonia, see Grew, Anat. of Plants, tab. 1 9, fig. 4, a and bj es- 

 pecially a< we are told by a modern author, who has published many microscopical observations on the 



