Rev. P. Keith on the Pith of Plants. 79 



to it. The stem measured seven inches in length, with three or 

 four leaves. The root measured three or four inches in length, 

 with many lateral fibres ; and the diameter of the plant, at 

 the widest, was about one eighth of an inch. In taking a lon- 

 gitudinal section of a portion of the root and stem so as to 

 pass through the collum, which could not be mistaken, as the 

 lobes never rise above the level of the soil, it was evident that 

 the pith, sti-ictly cellular and under the aspect of a fine thread, 

 descended into the root, without any node or interruption or 

 breach of continuity whatever, and without any appreciable 

 difference beyond that of colour. Above the collum it w^as of 

 a deep red; below it was of a pale green. If any doubt re- 

 mains in the mind of the reader as to the accuracy of this 

 statement, I shall be very glad if he will have the goodness to 

 repeat the experiment on a seedling of the same species and 

 of the same age, and to say what he thinks of it then. With 

 regard to myself, I hold it to be a most satisfactory proof of the 

 existence of a pith in the descending axis even of exogens. It 

 may be seen equally well in the root of seedlings of the oak 

 and ash, but without the pecuUarity of the red and green 

 colours. 



2ndly. The other debateable point on the subject of the pith 

 is as follows : 



Does the pith, after having reached its maximum of dia- 

 meter and parted with its specific juices, ever shrink further 

 in its dimensions, whether by the generation of longitudinal 

 fibres within it, or by pressure from without, or by any other 

 cause ? In the earlier days of botanical inquiry, it was the 

 opinion of phytologists that the pith is obliterated with age, 

 or at least much diminished in its diameter. This opinion 

 was advocated by Mirbel in his ^ Physiologic Vegetale*, where 

 he not only states the fact, but explains how^, in his opinion, 

 the change is effected, — that is, by being converted, first, into 

 longitudinal tubes and then into wood. But on the contrary, 

 there are botanists who contend that '' the pith undergoes no 

 change after the end of the first year of its growth -'' at which 

 period it may be said to have become distinctly cellular, and 



* Liv. III. 



