

Notes on Bark Structure 153 



almost completely surround the bast bundle. The paren- 

 chyma cells are nearly as wide as the bast fibers; they never 

 develop into mucilage cells and are never sclerotic. The sieve 

 tubes are broader than the parenchyma, and their ends are 

 slanted and contain several sieve plates. Some of the pith 

 ray cells show between fiber bundles, a trace of sclerotic wall. 



Summary : 



Tangential bast fiber bundles surrounded by crystal 



cells. Pith rays broad and wider toward the outside. 



Sieve-tubes with ladder-like end plates; crystals large, 



prismatic. Crystal-aggregates in primary pith rays; cork 



layer evenly small-celled. 



Tilia 



Leguminosae. 



Outer bark. The phellogen develops from a fourth to 

 sixth row of cells below the epidermis in the primary bark of 

 Robinia; in Gleditsia from the second or third row and in 

 Gymnocladus, from the row immediately adjoining the epider- 

 mis. The first formed periderm expands with the growth in 

 thickness of the twig for several years in Robinia. The super- 

 ficial periderm rarely reaches large dimensions despite its long 

 persistance. In both Robinia and Gleditsia it consists of thin- 

 walled spongy cork while in Gymnocladus it is thick walled. 

 The cork cells are for the greater part cubical or only slightly 

 flattened in form. 



Middle bark. The hypoderma of collenchyma cells is 

 either wanting or is only slightly developed. The thin-walled 

 closely fitting parenchyma cells of the primary bark begin, in 

 very young internodes, to become sclerotic between the bast 

 fiber bundles and thus close the thick walled cells into a ring. 

 This feature is characteristic of the Leguminosae, as also is 

 the presence of very few crystals in the primary bark. Single 

 crystals are formed in the region of the stone cells and in the 

 Ihin walled parenchyma. 



Inner bark. A characteristic of the Leguminosae is that 

 the development of the pith rays is never influenced by the 

 bast strands. The latter form concentric layers in Gymno- 

 cltulns and Gleditsia. The crystal cells are thin-walled in 

 Robinia, and partly sclerotic in Gleditsia and Gymnocladus. 

 The soft bast of the two last mentioned genera is composed of 



