SECONDARV THICKENING. NORMAL DICOTYLEDONS. 525 



' narrow sieve-tubes, which are very isolated, and therefore easily overlooked, and are 

 accompanied by narrow, elongated cells. 



Finally, an arrangement deviating further from the usual rule is represented by the 

 bast of the roots of Taraxacum. Narrow, concentric, annular zones containing the 

 tubes, here alternate regularly with similarly arranged, broad zones of large-celled 

 parenchyma, which are on the average about sixteen layers of cells in thickness. The 

 zones containing the tubes consist of narrow cells, numerous milk-tubes, and scanty 

 sieve-tubes ; the large-celled zones consist of thin-walled cells, which are placed in very 

 regular radial and vertical rows, corresponding to the original cambial form and 

 arrangement. At numerous points the radial series of these elements pass through the 

 zones containing the tubes, so as to interrupt them, without however consisting of 

 special ray-parenchyma distinct in form from that of the annular zone. The roots of 

 Chelidonium and Papaver are intermediate in structure between those of Taraxacum, 

 and of the first category. 



Sect. 163. In plants which have laticiferous tubes (p. 186), the non-articulated 

 ones may be absent from the secondary bast ; at least I did not find them in it in 

 Vinca, Asclepias curassavica, and the Euphorbias; in most cases they are present, and 

 as regards the articulated tubes especially, this is always the case so far as investigation 

 extends; they are then characteristic companions or representatives of the sieve- 

 tubes. The large non-articulated tubes in Ficus, Madura, and Morus follow singly 

 the lines of sieve-tubes. The articulated, usually reticulate tubes form groups 

 together with the sieve-tubes, which anastomose, both within each individual 

 strand of bast, and with those of neighbouring strands, by means of connecting 

 branches. In the bast of the Papayacese the net of milk-tubes has a relatively 

 scanty development, at least in comparison to its complexity in the wood. In the 

 other plants belonging to this category the milk-tubes are always relatively very 

 numerous, as is especially conspicuous in the shrubby stems (Sonchus pinnatus and 

 Campanula Vidalii), and in the roots of Cichoriaceae, Campanulaceae, and Pa- 

 paveracese : as their number increases the sieve-tubes become proportionately 

 reduced. In the strands of bast of the roots of Cichoriaceae (Lactuca virosa, 

 Taraxacum) only scanty, narrow sieve-tubes are present, as has already been 

 mentioned above ; in the secondary bast of the root of Platycodon grandiflorus, I 

 did not find the latter at all, though I do not wish to assert that they are entirely 

 absent. This mutual representation appears in the most striking manner in the bast of 

 the roots of Papaveraceae : Papaver Rhoeas and Argemone mexicana have only very 

 isolated sieve-tubes side by side with the highly developed net of laticiferous tubes ; 

 in Chelidonium majus the former are more numerous, although the milk-tubes pre- 

 dominate ; Glaucium luteum has no milk-tubes, but, on the other hand, has large 

 groups of sieve-tubes. 



Sect. 164. The occurrence of protogenetic secrefory passages in the soft-bast 

 has already been to some extent noticed in anticipation in Chapter XIII. They only 

 occur in those plants and parts of plants which also possess them in the primary 

 tissues, and by no means in all even of those. Their position appears to be always 

 within the bundles, not in the medullary rays. 



Among the Dicotyledonous families in question their occurrence in the 

 secondary bast of stem and root in the Terebinthaceae, Burseraceae, and Clusiaceae 

 has already been mentioned above. So far as is known they are present in the 



