LA.TICIFEB0U8 VESSELS OF HEVEA. 573 



articulated laticiferous cells, the transition having been effected 

 through forms like Johannesia, where certain of the sacs grow to 

 a great length*. His suggestion may be provisionally accepted ; 

 and I would further call attention to the still clearer relationship, 

 already noticed, between the sacs and the true laticiferous 

 vessels. Combining this view with that of Dr. Pax, we arrive at 

 the conclusion that in the Euphorbiaceae forms with closed latici- 

 ferous sacs have given rise, on the one liand, to those with typical 

 laticiferous cells (e. g. Euphorbia), and, on the other hand, to 

 those with true vessels, produced by cell-fusion (e. g. Manihot, 

 Hevea). This conclusion appears well worthy of being tested by 

 further observation. The Papaveracea? afford an especially in- 

 teresting series of cases, showing how the transition from the 

 sacs to the vessels may have taken placet. 



In Sanguinaria the characteristic red sap is contained in large 

 sacs scattered through the parenchyma, which are either isolated 

 or form continuous longitudinal series. In the root the sacs are 

 roundish or shortly cylindrical ; while m the stem and petiole 

 they are elongated. Similar organs occur in Glaucium, Mac- 

 let/a, &c. 



In Chelidonium we find these sacs replaced by vessels ; but 

 the transverse walls are partially persistent throughout life, 

 and the vessels do not form a network. The members of these 

 vessels show the same differences of form in stem and root as 

 do the sacs in Sanguinaria. 



Lastly, in Papaver the members of the vessels are completely 

 fused, no trace of the transverse walls usually remaining, and are 

 further laterally connected to form a complex reticulate system. 



The Aroideje contain an equally instructive series of forms, for 

 which reference may be made to De Bary, I. c. 



Such examples certainly appear calculated to throw great 

 light on the mode in which laticiferous vessels first originated ; 

 and if, in the Euphorbiace«,we should also be able, in accordance 

 with Dr. Pax's suggestion, to trace back the inarticulated latici- 

 ferous cells to their origin from secretory sacs, we should have 

 reached the very interesting result that the two forms of latici- 

 ferous tissue, so different in their typical characteristics, have 

 yet been derived by modification proceeding along two distinct 

 lines, from originally similar histological elements. 



* L. c. p. 410. 



t On the Papaveracea? and Aroidea>, cf. De Barr, I. e. pp. 190, 435, & 436. 



LINN. JOUBN. — BOT.VNT, VOL. XXI. - L 



