FIBROVASCULAR TISSUES: RAYS 69 



angiosperms.) There is good reason to believe that the "bars of 

 Sanio" are an original and characteristic feature of the organization 

 of the wood of the Coniferales and allied groups. The tracheids 

 shown in the figure differ from those of the cordaitean forms by 

 their periodic variation in size, those laid down in the beginning of 

 the annual increment being of larger caliber than those coming 

 into existence toward its close. The late tracheids are distinguished, 

 as indicated in earlier pages, by their tangential pits seen in profile 

 in the radial section. Turning our attention now to the ray 

 itself, we see at once from the figure that a considerably greater 

 degree of complication is present than that exemplified by the 

 similar structure in the case of the Cordaitales. Manifestly the 

 radial elements are of two kinds. First there are the cells which 

 constitute the central region of the ray and which in life are char- 

 acterized by protoplasmic and other contents. This situation is 

 indicated by the copious simple pits which ornament the vertical 

 and horizontal walls of the cells. The strong pitting is also a clear 

 feature of difference from the Cordaitales where the walls of the 

 ray cells are in general thin and unpitted. Laterally the central 

 elements of the rays are related to the tracheids by means of very 

 large, somewhat angular pits. The second type of element char- 

 acteristic of the ray in Pinus is likewise distinguished by the nature 

 of its pitting. All the pits seen on the walls, whether horizontal, 

 vertical, or lateral (related to the tracheids), belong to the bordered 

 type. The cells of the ray possessing this peculiar organization 

 are typically marginal in position and, naturally, are quite without 

 protoplasmic contents, except in the early stages of development. 

 Such tracheary elements in coniferous rays are commonly desig- 

 nated marginal tracheids or simply marginal cells. They obviously 

 permit the easy movement of water in the radial direction in those 

 woods characterized by their presence. 



In the tangential view presented in b, Fig. 49, it is clear that 

 there are two types of rays namely, narrow uniseriate ones and 

 broader ones tapering at either end to the uniseriate condition. 

 The former are known as linear rays and the latter as fusiform rays. 

 It is evident from the figure that the ray of greater width is char- 

 acterized by the presence of a partly occluded cavity, a resin canal. 



