604 R. J. HARVEY-GIBSON AND MINNIE BRADLEY ON 



occupies the centre of the bundle, and the concentric character is found to be due to 

 the insertion of an axillary branch. Such concentric vascular cords also occur at 

 the Lower ends of the leaf-trace bundles of many rhizomes of Monocotyledons, e.g. 

 Iris germanica, Cyperus aureus, Carex arenaria, Papyrus, etc. (5, 6). 



4. Folding in Cell Walls. 



.Many of the cortical cells of Corydalis racemosa as well as of the parenchyma 

 surrounding the vascular bundles show marked folding of their walls. Often these 

 cells form a continuous layer, suggestive of an endodermis. Frequently such cells 

 are unequally distributed, e.g. the dorsal and lateral surfaces of the bundles are 

 destitute of them, while the ventral side may have a layer three or four cells thick. 

 This irregularity of distribution, however, scarcely suggests an endodermis. In 

 Corydalis solida the cells with folded walls are often restricted to one layer round 

 each vascular cord, which may have led to the conclusion (3) that this layer was an 

 endodermis and that Corydalis solida was polystelic. Detailed investigation shows 

 th'at this view is untenable. In Argemone ochroleuca cells with folded walls occur 

 not only round the vascular bundles but also in the epidermis and cortex, while 

 in Dicentra formosa practically the entire medulla consists of such cells, as well as 

 the cells in the more immediate neighbourhood of the vascular bundles. In Adlumia 

 cirrhosa and Eschschohia californica folds occur on the walls of some of the cortical 

 cells as well as in those near the vascular system. Leger and De Bary also suggest 

 that these folds are caused by the limiting nature of the tissue in which they are 

 found, but this theory does not account for the folds in the medullary cells of 

 Dicentra formosa nor for those in the cortical cells of Corydalis racemosa. 



5. The Occurrence of- Reticulate Secondary Thickening in the 

 Walls of the Secondary Xylem. 



In by far the majority of plants the walls of the vessels and tracheids of the 

 secondary xylem are pitted, but in Papaver there is a preponderance of the reticulate 

 type of secondary thickening, while P. pilosum, P. lateritium, P. rupifragum, P. 

 orientale, P. nudicaule, and P. alpinum show reticulate elements, to the exclusion of 

 all pitted forms. Reticulate elements are also found in Meconopsis cambrica and 

 Chelidonium majus. Similarly, the xylem of the Crassulacese also shows reticulate 

 thickenings in the secondary wood, though the stems in that order are much more 

 lignified than in the Papaveracese. 



G. The Occurrence of Bordered Pits. 



Bordered pits occur in the xylem elements of Bocconia microcarpa similar 

 to those found in other dicotyledonous woods, e.g. Cheilanthus arboreus, 

 Vitis, etc. 



