1 94 LATICIFEROUS TUBES. 



special stress upon the coalescence of originally separate cells, since he places ail 

 milk-tubes among his ' cell-fusions/ and only cites as cases where the cells do not 

 coalesce to tubes, those of Chelidonium, in which plant he overlooked the perforation 

 of the tepta, and those of Sanguinaria, which must be entirely excluded from the 

 category of milk-tubes. All investigators acceded in the main to Unger's view. 

 Dippel and Hanstein made thorough investigations, which proved it clearly for many 

 cases (articulated tubes). Schacht had already published at an earlier date ^ an excel-, 

 lent history of the development of the tubes of Papaya from coalescing cells of the 

 meristem, and greatly shaken thereby his own view repeatedly proclaimed since the 

 above-cited work of 1851, according to which the milk-tubes are no special form of 

 tissue at all, but only ' Bast cells,' i. e. sclerenchymatous bast fibres, filled with latex I 



All the later authors mentioned, who expressed their views on the subject, 

 extended the above theory of coalescence to all milk-tubes, both articulated and non- 

 articulated. The first objections to this are to be found indicated by Hartig, but have 

 recently been more clearly brought forward by David ". As may be seen in the above 

 statements, the two sorts of tubes should be treated separately. 



For the articulated tubes it can no longer be doubted, after the excellent descrip- 

 tions of their development by Schacht, and particularly those by Dippel, that they 

 arise, as above described, by coalescence of cells. Without the development having 

 been traced step by step, this follows in the case of the tubes connected into a net 

 from the fact that in an earlier stage there are only simple cells of the meristem in the 

 position afterwards occupied by the network. The fact may be proved most- clearly 

 in the secondary cortex of the Cichoraceae. In the tubes of Chelidonium, which are 

 not connected into a network, the limits of the original cells remain partially preserved 

 through life. 



The non-articulated tubes offer much greater difficulties. Most authors since 

 1846 have simply extended to them the same history of development as was observed 

 for the articulated tubes ; only Dippel and David attempted to get to the bottom of 

 the question by direct observation. Dippel followed the tubes, in Ficus Carica and 

 Euphorbia splendens, into the youngest meristem of the growing point, and close to 

 the latter he found here and there septa in the tubes : he found such septa also in 

 occasional preparations of old tubes of Euphorbia Cyparissias, Asclepias curassavica, 

 Nerium Oleander, and Vinca minor, and concludes from this that they arise by 

 coalescence. 



David arrived at a quite different result for the families named in the title of his 

 dissertation, of which he investigated the following species specially: Euphorbia 

 splendens, Caput-medus«, Lathyris ; Ficus elastica, Carica ; Nerium Oleander, and 

 Hoya carnosa. According to him each non-articulated milk-tube is one cell, ' a milk- 

 cell,' originating at an early stage by the elongation of one single cell of the meristem, 

 which branches, and thrusts itself between the elements of the surrounding tissue. All 

 the branches of each of these cells end blind and closed : the cell may grow to a great 



' Monatsbr. d. Berliner Academic, 1856, I.e. 

 ^ [F. O. Bower, on Gnetum, Q. J. M. S. 1882.] 



' Ueber die Milchzellen der Euphorbiaceen, Moreen, Apocyneen, und Asclepiadeen ; Dissert 

 BresUu, 1872. 



