TISSUES AND SIMPLE ORGANS. 



91 



of a raj in the medulla and in the primary cortex or at some dis- 

 tance from them (" primary " and " secondary " medullary rays), 

 the direction of elongation (stretching) of the cells, etc., produce 

 many changes in the appearance of the plant-structure. These and 

 similar anatomical characteristics are also of special interest to the 

 palaeontologist in determining the probable systematic position of 

 fossil-plants. Sometimes the connection between medullary ray- 

 tissue and wood-parenchyma is very distinct and forms a system, 

 when, for example, the wood-parenchyma cells form transverse 

 connecting-bands between the medullary rays. There is anotlier 

 phenomenon which is of frequent occurrence and which must 



Fig. 56. — Radial longiludiual section of the vascular bundle in the stena of 



(Enothera odorata. 



c. Cambium; g, primordial vessels. The different forms of vessels will be recognized from 

 previous descriptions. (After Haberlandt.) 



receive special mention, though its significance has only been ap- 

 proximately determined from a physiological standpoint, namely, 

 that the dead tracheal system and the living cells (medullary rays 

 and wood-parenchyma) form such "an intimate anatomical relation- 

 ship as to enable tlie interchange of fluids between them (assisted 

 by the cell-pores). Further, we notice an increase in the length of 

 the mechanical elements of the woody body in a direction from the 

 younger rings to the older rings (hence from without inward). 

 Upon this perhaps depends the apparent torsion of stems, that is, 

 the tangentially slanting arrangement of the woody elements. 



In regard to the vessels, a radial longitudinal section shows the 

 variable structure of the perforated transverse septa (remnants of 



