140 D. H. SCOT*. 



these vessels in the cambium of Taraxacum officinale and 

 Podospermum. He gives figures of stages in which the 

 cross-walls are still present, and also makes the interesting 

 statement that the communication between the vessels by 

 means of lateral branches is established before the cross- 

 walls are absorbed. He sums up his results in the follow- 

 ing words : — " The laticiferous vessels of both plants arise 

 from the fusion of carabiform or sieve-cells,^ which are 

 situated one over the other or side by side ; the fusion is 

 determined by the conversion of the membranes of the cells 

 undergoing amalgamation into pectose." Vogl, as shown 

 by his fig. 4, plate ii, confused the young laticiferous vessels 

 with sieve-tubes. 



The statement of Trecul (to whose researches I shall 

 return below) that a communication exists between the lati- 

 ciferous vessels and the tracheae called forth a renewed in- 

 vestigation of these tissue forms. Hanstein's prize essay^ 

 (1864) contains a very full treatment of the subject, in 

 which, however, the distribution and other anatomical 

 conditions are chiefly regarded. As regards the develop- 

 ment, Hanstein was an unconditional supporter of the theory 

 of cell-fusion. But he also failed to observe directly the 

 successive stages of development. With special reference to 

 the laticiferous vessels of the Cichoriacese, Campanulacese, 

 and Lobeliacese, he says that " the fusion of these cells 

 generally takes place at such an early stage that, owing to 

 the delicacy of the parts which still prevails, it escapes 

 direct observation. "^ He adds, however : " The arrange- 

 ment of the sac-shaped trunks of the vessels corresponds 

 exactly to that of the neighbouring cells, while the length 

 of the latter can often be recognised in the still distinguish- 

 able joints of the sacs." A little further on he gives the 

 grounds of his opinion still more clearly. He says -^ " On 

 the other hand, the origin of these vessels from chains of 

 cells finds its repeated proof, on the one side, in the com- 

 parison with those of Carica, and on the other in the Papa- 

 veracese,^' &c. We thus see that Hanstein^s view, like that 

 of Unger, is supported rather by analogy than by direct 

 observation. The argument from analogy had become, of 

 course, much stronger since the publication of Schacht's 

 observations on Carica. How incomplete the researches 



^ Vogl also uses the term " Leitzelleu." 



- ' Die Milcbsaftgefasse und verwandlen Organc der Riude,' Berlin, 

 1SC4. 



■' Loc. cit., p. 14. 

 * Loc. cit., p. 15. 



