12 Weiss, Phloem of LepidopJdoios and Lepidodendron. 



groups of phloem cells in LepidopJdoios which become 

 disorganised are not generally compressed by the firmer 

 tissues or by pressure during mineralization, but form wide 

 and rounded passages, often compressing the surrounding 

 tissues and showing that a force has been exerted from 

 within the lacunae A similar phenomenon may be noted 

 where the phloem groups have become partially or entirely 

 disorganised in Lepidodendron selaginoides, as can be seen 

 from Hovelacque's Figs, i and 2 on Plate IL of his memoir 

 on this plant. 



One further point arises in connection with the phloem 

 of the stem, and that is, the question whether any part of 

 it is of a secondary origin. The cell divisions seen in 

 the cambial layer tend to show that it does add a little 

 to the phloem. Fig. 2 seems to show in the case 

 of some cells towards the left hand side that they have 

 been derived from the cambium. While some of the 

 phloem elements near the outside seem to be compressed 

 very much in the same way as in dicotyledonous plants 

 with secondary thickening, where they ultimate!}- form 

 hardened masses of tissue described as keratenchynia by 

 Tschirch.^ It seems to me very likely that it is elements 

 of this kind which have been described in the Lepidodendra 

 sometimes as bast fibres, sometimes as by Bertrand^ and 

 Hovelacque^ as latex cells. They may be readily seen on 

 the outside of the phloem in Psilot?iin, and one would 

 expect them even more naturally in a stem in which 

 .secondary thickening has taken place. Should they be 

 cells of this nature it would be more easy to explain their 

 absence, or the uncertainty of their presence, in certain 



' Tschirch A. Angewatidte PJlanzenanatomie, 1889. 

 ^ Bertrand. " Remarques surle Lepidodendron Harcourlii dc \Vitham." 

 Trav. et Metn. des Facultis de Lille, 1891. 

 ' Hovelacque M., loc. cit. 



