igii LATICTFEROUS TUBES. 



' In Asclepiadese and Apocynene (apparently also in Ficus) the same seems to be 

 the case also in the apex of the root of the germinating seed. The tubes are much 

 narrower and difficult to follow. In the apex of the root they are distributed 

 throughout the cortex. My observations on them are in other respects still very 

 incomplete.' 



The origin of the above-mentioned milk-tubes in the secondary bast, which is 

 formed from the cambium, in Ficus, INIorus, Broussonetia, INIaclura, and Nerium, is 

 not explained by the above observations ; and I am unable, either from published 

 accounts, or from my own observations, to propound any well-founded view whether 

 they arise as branches of those in the primary cortex, and make their way into the 

 secondary bast, or whether they are successively formed anew from the cambium, and 

 in that case are in no direct connection with the primary tubes. INIaclura aurantiaca 

 may be especially recommended for the further pursuit of this question. 



However great the differences in development, above noticed, between the non- 

 articulated and the articulated tubes, still the frequently abundant branching of single 

 members of the latter, and perhaps the coalescence of branches of the former to 

 form anastomoses in the nodes, constitute a transition between the extremes of both 

 categories; and even without these, other common points of structure and dis- 

 tribution, and of functions as indicated by the latter, would certainly lead to 

 uniting both as one sort of tissue. 



Brief allusion has above been made, in accordance with the views of Dippel and 

 Hanstein, to their near relations to the sieve-tubes : this subject will be returned to in 

 Chap. XII. These relations are mainly physiological, and topographically-anatomical. 

 The assertions as to close histological relations between milk- and sieve-tubes are, I 

 believe, wrong, or at least exaggerated ; for instance Yogi's assertion that both the 

 resin-passages of the Convolvulacese, which do not belong to this category, and the 

 true milk-tubes, e.g. in the Campanulaceae, develope from sieve-tubes, or at least may 

 do so. Such a development never occurs. Again, it is especially stated by Dippel, 

 both that the septa in articulated tubes — e.g. in Chelidonium and Papaver — are 

 perforated like sieve-pilates ; and also that the lateral walls (e. g. Papaver, Cicho- 

 raceae, Carica) are often provided with sieve-plates, and that thus there are in these 

 cases intermediate forms between sieve-tubes and articulated milk-tubes. I could never 

 find this phenomenon, but rather only smooth, delicately outlined pits on the lateral 

 walls, or wide, sometimes grouped perforations. Schmalhausen's investigations on this 

 point also gave the same result. Further, Dippel's drawings of these structures show- 

 little similarity to true sieve-plates of dicotyledonous plants. The septa often resemble 

 them, it is true, inasmuch as the}' are perforated by more than one pore ; but the pores 

 are coarse, wide, and irregular, and have none of the other characteristics of structure 

 of sieve-plates (comp. Figs. 80, 81). On the contrary, the sharp difference of structure 

 between the sieve- and milk-lubes is always particularly clear, where one would 

 a priori the most expect to find intermediate forms, i. e. where both are closely side 

 by side, as is the case in the secondary bast. 



The view, so long held, as to the near morphological relations between milk- 

 tubes and sclcrenchymatous fibres — 'bast-cells' — has been above refuted. It was 

 excusable that Mirbel at the beginning of this century confused the two organs, and 

 came in 1835 to the conclusion that all the sclcrenchymatous fibres of the secondary 



