LATICIFEROUS CELLS, DEVELOPMENT. I95 



length, in E. splendens, e. g. more than i zmm in E. Lathyris, the length of an inter- 

 node plus the leaf belonging to it, that is about 20 cm. As the plant grows, new 

 milk-cells are formed in the growing point of the stem : the tubes of the leaves are 

 only branches of those which traverse the stem. It is obvious that this view scarcely 

 differs except in name from Schacht's theory of bast cells. David's chief proof of his 

 statement was obtained by him by macerating sections of the apical meristem with 

 potash, and then teasing out from them the young milk-cells, which are at first short 

 and spindle-shaped, but gradually become elongated and branched. In non-macerated 

 sections also he was able to find the required early stages. The examination even 

 of the mature specimens shows that David's view is impossible, for, as above shown, 

 the tubes, e. g. of the Euphorbias, can be followed for any distance, and numerous 

 blind peripheral ends of branches may be found in leaves, cortex, and growing points, 

 but never a completely closed tube which is of less length than the whole plant. 

 Were the tubes originally formed in succession as single completely closed cells of 

 the growing point, they must have coalesced with one another to form the mature 

 tubes. 



As a matter of fact those solitary spindle-shaped initial cells of the milk-cells do 

 not exist. The tubes run continuously into the furthest meristem of the growing 

 point, their ends can be followed to within 6-8 cells of the extreme apex : their 

 course is variously curved both in radial and tangential direction between the de- 

 veloping parenchymatous cells; longitudinal sections must thus cut off portions of them, 

 which are roundish, or spindle-shaped, or cylindrical, and look exactly like cells of 

 such form, especially if the section be very thin and transparent, or if the delicate, 

 very readily swelling membrane be swollen by maceration in potash. David's young 

 milk-cells are such sections of tubes : they were represented by Dippel, but were 

 rightly explained. 



Dippel's view does not rest, like that of David, on mistakes of observation, which 

 might easily be avoided : it is, from analogy with articulated tubes, extremely probable : 

 still I was unable, though I investigated the matter repeatedly, with every prejudice in its 

 favour, to find the view confirmed by observation. If one examines the ends of stems 

 in species of Euphorbia, Stapelia, or Ficus, which are elongating, but before secondary 

 thickening begins, the last ends of the tubes and their branches always extend, as has 

 been repeatedly stated, into the furthest meristem of the growing point, and of its 

 youngest leaves and branches, and I was never able to observe in their cavity traces 

 of septa or of any coalescence of originally separate cells. Where septa appeared to 

 be present in the young ends of tubes, and this was not rarely the case, continued 

 and careful observation of the preparation always led to the conclusion that these 

 were not within the tube itself, but belonged to cells above or below it. The state of 

 things above indicated is best seen in radial longitudinal sections, which have become 

 quite transparent by maceration for one or several days in very dilute solution of 

 potash, without very considerable swelling of the cell-membranes, and which are 

 thick enough to allow of following the tubes for a considerable distance. Accordingly 

 I can only explain the septa, which Dippel represents as present in the young ends 

 of milk tubes, as walls outside the tubes, or perhaps also as the contiguous walls of 

 two tubes cut obliquely. The septa described by him as occurring here and there, I 

 also have certainly been able to find not uncommonly at the nodes (but only there), in 



