190 PIIYTOTOMY. 



the pith detached from the wood, until at last it entirely dis- 

 appears, and leaves the stem hollow, as we commonly observe 

 in the umbelliferous plants ; or vacant spaces occur, and the 

 pith is found only about the joints. Sometimes these vacan- 

 cies are divided by regular partitions, as we observe in Jun- 

 cus glaiicus, in Cictita virosa, in trees of the Walnut kind, 

 and in Rose bushes; in which last, from the regular con- 

 struction of these spaces, a connected cellular texture may 

 be remarked. 



The pith vanishes in the hardest woods, because these press 

 ever more and more towards the centre, and, by uniting with 

 the cells of the pith, render them at last completely indistin- 

 guishable. 



300. 



As the layers of the stem become united in the joints, and 

 the primitive vessels are there crowded together and take a 

 different direction, the pith cannot be supposed to proceed 

 through the joints unchanged. There are indeed no pecu- 

 liar partitions, which intersect the cavity of the pith in the 

 joints ; but it is so intermingled with the other parts, that its 

 continuation is evidently interrupted. But the stronger 

 branches are excepted from this remark ; for they are knotted 

 and push out new shoots, without our being able to observe 

 the interruption of the pith. The forming sap hei-e pushes 

 into the space between the bark and the inner-bark, to fonn 

 a reservoir from which new shoots may be unfolded. 



301. 



From the interruption of the pith in the joints, it follows, 

 that this substance is by no means so essential to the produc- 

 tion of the fruit, as some naturalists have believed. And this 

 idea is still further opposed by the fact, that in the Syngene- 

 s\a necessaria we observe the perfect seed only in the circum- 

 ference ; whilst in the middle, where the pith might have had 

 some effect, we see the seed either imperfectly formed, or en- 

 tirely wanting. There are also a gi'eat many trees, which, 

 without having any peculiar pith-cavity, yet produce rich 



