198 D. H. SCOTT. 



The observation of the facts just stated is sufficient to show- 

 that the laticiferous tissue of Manihot is essentially different 

 from that of Euphorbia^ which has hitherto been regarded as 

 iu this respect typical of the order. The whole appearance of 

 the tubes is quite different from that of laticiferous cells. 

 There are no long^ blindly ending branches^ and the general 

 character of the tissue is closely similar to that of the latici- 

 ferous vessels in the Cichoriacese. That cell-fusion takes place 

 throughout the laticiferous tissue of Manihot is obvious even 

 on inspecting it in its mature state, from the innumerable per- 

 forations which are present in the lateral walls. It remained, 

 however, to be proved whether the individual tubes themselves 

 are derived each from a single cell, or whether they are the 

 product of the fusion of longitudinal series of cells. 



The investigation of radial sections extending to the cam- 

 bium, from a stem in which secondary thickening is in active 

 progress, leaves no doubt as to the true character of these 

 tubes. In the innermost zone of laticiferous tissue, in the 

 immediate neighbourhood of the cambial layer, the transverse 

 walls in the vessels are easily seen. In some the walls are still 

 completely preserved, in others there are small perforations, 

 through which the contents are already continuous, a decided 

 constriction marking the point where they traverse the par- 

 tially absorbed wall. In somewhat older stages only a narrow 

 rim of cellulose remains, projecting into the lumen of the 

 vessel. Tangential sections which exactly pass through the 

 youngest region of the secondary cortex are still better adapted 

 to show the mode of development of the tissue in question, as 

 every stage of perforation of the transverse and longitudinal 

 walls, leading to the formation of the continuous network of 

 tubes above described, can be observed (fig. 3). The cells of 

 which the laticiferous vessels are built up, are of about the 

 same proportions as neighbouring cells of the phloem paren- 

 chyma.i 



' Methylene blue, which stains the cellulose walls only, is very useful in 

 demonstrating tlie various stages of perforation. I have to thank my friend 



