146 VEGETABLE ORGANOGRAPHY. 



row of fibres, isolated from one another, and dispersed in 

 the pith upon tlie outer border of the canal. Hedwig 

 designated them, in his first works, by the flame of Vasa 

 fibrosa ; and I called them Medullary Fibres {fihrce 

 medullar es ; fibres medullaires ) . Lastly, in a small 

 number of Exogens, these same fibres, instead of being 

 arranged circularly, are scattered throughout the whole 

 pith ; we remark this easily in the stem of Ferula com- 

 munis (PI. 2, fig. 3); and Mirbel has also observed it in 

 that of the Marvel of Peru. The stems of Ferula have the 

 pith very large, intermixed with scattered fibres, and the 

 woody body scarcely visible, as happens in most annual 

 stems ; so that, at first sight, one would be inclined to 

 take them for Endogens. These Medullary Fibres 

 sometimes become coloured, when the young stems are 

 soaked in coloured water, which shows that they give 

 passage to the sap : the cellular part never becomes 

 coloured in these experiments. This was likewise the 

 result, both from trials by the Jesuit Serrabat, who, 

 under the name of Delabaisse, has published a Disser- 

 tation upon the circulation of the Sap, and from my 

 own experiments upon this subject. 



The Medullary Sheath (vagina medullar is ; etui 

 medullaire) is a woody layer which immediately sur- 

 rounds the pith. Du Petit-Thouars observes, that it 

 seems to form a cylinder continued from the top even to 

 the bottom of the tree ; but that, like the pith, it is com- 

 posed of as many parts as there are shoots. It is a layer 

 of woody fibres, in which are found Tracheae, capable of 

 being unrolled, not only in the first year, but, as Mirbel 

 and Du Petit-Thouars have observed, in very aged 

 trunks. 'In several trees this Medullary Sheath pre- 

 serves, even in the old branches, a green colour, which 

 shows that it is still endowed with vitality ; Senebier has 

 seen this in Phytolacca, and I have remarked it also in 



