l88 LATICIFEROVS TUBES. 



tubes, and are in immediate contact with these ' : further, that in the xylem of the 

 stem of the Papayaceae the laticiferous tubes are directly and firmly attached to the 

 large vessels, sometimes throughout their length, sometimes by single ends of their 

 branches '^ : again, that a similar relation exists between the laticiferous tubes which 

 accompany the vascular bundles of many Aroidese and the tracheae which belong to 

 them '- Finally, it is an indubitable fact that in sections through plants having latici- 

 ferous tubes, numerous vessels are often to be found filled with coagulated masses, 

 which appear very similar to the coagulated latex of the plant, and which also 

 have its characteristic colour, e.g. in Chelidonium reddish yellow*. This latter 

 phenomenon often occurs very conspicuously in roots, and under conditions which 

 do not allow of the idea of a flow of the latex from a cut surface. Trdcul concludes 

 ■from this series of facts, observed by him in many cases, that in all plants with 

 laticiferous tubes at least single branches of the tubes come into direct contact with 

 tracheae, and open communication is set up by perforation of single portions of the 

 wall at the points of contact ° He even states that he has directly observed the 

 points of perforation, e.g. in Lobelia laxiflora. Other observers, among whom I 

 must place myself, according to my investigations up to the present time, have been 

 unable to find such contact and communication of milk-tubes with the tracheae, 

 with the exception of the above-mentioned cases of the Aroideae and Papayaceae: on 

 the other hand, they have seen that where branches of milk-tubes run from the 

 cortex to the pith, they take a course by the medullary rays through the woody or 

 vascular ring. Trdcul's statements accordingly require further proof: in the first 

 place, that on the general contiguity of milk-tubes and vessels, and secondly, that 

 regarding the open anastomosis of milk-tubes into vessels, which should be confirmed 

 in the case in which it was observed by him, and, at all events, in the Papayaceae and 

 Aroideae. According to the present data these anastomoses occur very seldom, 

 and it is extremely difficult to prove them with certainty by direct observation, and to 

 distinguish them from non- perforated pits. If they do really occur, still it is not 

 proved that they are proper to normal, tissue, and not really pathological phenomena, 

 i. e. ruptures in the thin places of contact of the tubes, which arise in the same way, 

 through the pressure of the turgescent parenchyma, as the discharge of latex on 

 surfaces of section. The apparent presence of coagulated latex in vessels is evidence 

 at first sight of the existence of open pores : the normal existence of these is however 

 made the more, doubtful by the fact that, as far as investigated, the occurrence appears 

 to be quite irregular and inconstant, and that milky or resinous coagula are found in 

 vessels even in such plants as have no laticiferous tubes, but closed secretory cavities, 

 without any open connection with the vessels. 



If an anastomosis, or even a mere contiguity of milk-tubes with the tracheae, 

 does not occur in most cases, and I consider this most probable, there is then no ex- 

 planation of the occurrence of what appears to be coagulated latex in the latter : for 

 such an explanation it is however necessary, in the first place, to answer the question, 



' Compare, e.g. Hanstein, Milchsaftgefasse, Taf. IX. fig. 13 (Lactuca virosa). 

 ' Trecul, Ann. Sci. Nat. 4 ser. VII. p. 289, pi. 12 (1857) ; Comptes Uendus, torn. 45, p. 402. 

 ' Compare Hanstein, /. c. ; Van Tieghem, Structure des Aroidees, /. c. Taf. II. fig. i, pp. 6-8. 

 ' [Compare von Hohnel, Milchsaft in Tracheen, &c. Hef. Bot. Jahresbericht, 1878, 1, p. 30.] 

 » Compare especially Comptes Rer;dus, tom. LX (1867), p. 78, 



