Caprifoliaceae. 159 



peculiar phases of development. The epidermis and hypodermis 

 (Fig. 2 a) start to fall off in the 3rd year, through the destruction 

 of the underlying 2 — 3 layers of cells (Fig. 2b)\ the protective 

 function of the former is then taken up by the 1 — (5) layers 

 of large bark cells (Fig. 2 c) underneath, the walls of which 

 become woody. During the 4th year however, this protecting 

 layer is also partly thrown off, owing to the cells immediately 

 underneath being similarly destroyed ; the outer cells then 

 form a third protective layer by a strongly developed forma- 

 tion of cork. 



The diameter of the stem is about the same all over, 

 as the growth in thickness keeps pace with the decrease in dia- 

 meter, caused by the falling off of the several layers of bark. 

 Under the bark is the cambium (Fig. 2 d) and under this the 

 wood (Fig. 2 e), which yearly grows a little in thickness and 

 shows annual rings. The wood consists of vascular tissues, trach- 

 eids, libriform-cells, wood-parenchyma and medullary rays. 



The original transverse walls have the appearance, even 

 in the fully developed vessels, of a grate, consisting of very 

 thin, parallel staffs. The vessels lie scattered in the wood, 

 but not in such great numbers as the tracheids ; all kinds of 

 transitions to these latter are however found, and the circular 

 or oblong pores have the same trabeculate arrangement. 



The greater part of the wood consists of tracheids ; 

 libriform-cells and wood-parenchyma are rarely met with, 

 and are of quite subordinate importance. Between the pith 

 (Fig. 2g) and the wood are found a few layers of cells, which 

 both in shape and position form the transition between the 

 medullary cells and the tracheids; as the figure shows, their 

 walls have circular pores (Fig. 2 /). 



The numerous medullary rays consist of 4 — 8 radiant 

 rows of cells placed vertically above each other; they are 

 always well filled with starch, which can however also be 

 the case with the cells of the living bark. 



