out vessels is formed, the fibro-tracheids of 

 which have circvQar pits in their lateral walls, 

 but typical scalariform pits in the walls of the 

 overlapping tracheid ends. There is no in- 

 dication that such scalariform pits have arisen 

 from the fusion of rows of circular pits. 

 They are evidently a primitive featxire of the 

 tracheid and closely resemble the scalariform 

 pits of the secondary wood tracheids of 

 Drimys found in the vicinity of the pith; 

 yet only a slight modification of the tracheids, 

 with dissolution of the closing membranes and 

 borders of the scalariform pits, would com- 

 plete transformation into typically perforated 

 scalariform vessels. 



It is evident that, to the extent that ad- 

 jacent cells become specialized, and unlike in 

 shape, in size, and in function, as, for ex- 

 ample, the tracheal segment and an adjacent 

 prosenchyma cell of the angiosperms, the 

 scalariform pit must lose its alignment as an 

 intercommunicating structure. Conversely, the 

 circular pit is the more adaptable, and prevails 

 in the vascidar elements of more advanced 

 plant families as typified by the Compositse. 

 Just as might be hypothesized, the scalariform 

 pit is relatively more common in the vascular 

 elements of less specialized families included 

 in the Ranales. Probably the scalariform pit 

 prevailed in the early angiosperms, and is 

 even now being slowly discarded for the smal- 

 ler circular pit. It was, in case of the vessels, 

 first discarded on the lateral sides adjacent 

 specializing tracheids, ray-cells, wood-paren- 

 chyma, or, especially, fibro-tracheids and fibers. 

 The close relation between a vessel and the ad- 

 jacent element or elements is evident from the 

 fact that, for example, in Cheirodendron (Ara- 

 liaceae) four distinct types of pits communi- 

 cate respectively with prosenchyma, wood-paren- 

 chyma, ray-parenchyma, and with other ves- 

 sels. However, that the end-walls should pre- 

 serve the more primitive sculpture is quite in 



