134 IX. THE DICOTYLEDONOUS STEM HERBACEOUS TYPES. 



in full activity, and the vessels nearest to it (m") are manifestly 

 still in course of development. In exactly the same way in the 

 bast, or phloem, the incomplete passes over into the completely- 

 developed tissue, though here the transition is effected more 

 rapidly. In the bast we can see the sieve-tubes (v) with their 

 companion-cells, and between them parenchyma which we can 

 call bast or phloem parenchyma, in contradistinction to the 

 xylem parenchyma. In the periphery of the bast we can see the 

 protophloem groups (cbp) enclosed in parenchyma, and consisting 

 of narrow sieve-tubes and companion-cells. In the active sieve- 

 tubes the companion-cells are found on their cambial side ; they 

 are distinguished by their small radial diameter and the richness 

 of their contents. Phloem parenchyma is wanting in the bast of 

 Monocotyledons, but is well-nigh universal in Dicotyledons, so 

 that the case of Ranunculus in which it is not present must be 

 looked upon as exceptional in this respect. 



x If the cross-section has been taken from a twig in a proper 

 stage of development, we see between the cambium of the sepa- 

 rate vascular bundles (the fascicular cambium) the commencing 

 formation of the interfascicular cambium (ic). In the elements 

 of the ground-tissue, the conjunctive parenchyma, which are 

 interposed between the vascular bundles, and constitute the 

 primary medullary rays, we can see the interpolation of tangen- 

 tial division walls, so that streaks of cambium are formed which 

 connect the cambiums of the circularly-arranged vascular bundles 

 into a closed cambium ring. As the accompanying figure 

 (Fig. 49) shows, the method of formation of this interfascicular 

 cambium is in this case extraordinarily easy to follow ; and the 

 original contour of the ground-tissue cells which have undergone 

 division can be long recognised. The cambium ring, by means 

 of which Dicotyledons and Gymnosperms in general thicken, is 

 formed therefore of an alternation of fascicular and interfascicular 

 portions. The secondary tissues developed by the activity of 

 this cambium ring and upon its inner side are wood or xylem 

 tissues, those developed on its outer side are bast or phloem 

 tissues. A sharp limit between the primary and secondary 

 tissues is not distinguishable so far as the original bundle is 

 concerned ; but the interfascicular bundles, naturally, are wholly 

 secondary. The progress of lignifaction after cambial develop- 

 ment can be very beautifully followed by the phloroglucin and 

 hydrochloric acid method (see Chap. V., p. 78). 



