THE ANATOMY OF WOODY PLANTS 



that they manifest themselves in the most significant manner. 

 The genus Picea will perhaps serve best to illustrate the conditions 

 involved in the appearance of parenchymatous elements in the 

 secondary wood. Fig. 32 illustrates the structure of a transverse 

 view of the wood of the root of Picea canadensis. Parts of two 

 annual rings are represented. The summer wood is easily recog- 

 nized by the thicker 

 walls of the tracheids 

 and the tangential pits. 

 It passes abruptly into 

 the spring wood of the 

 next annual increment, 

 characterized by large- 

 sized thin-walled ele- 

 ments with entirely 

 radial pitting. The cru- 

 cial feature of the figure 

 is presented by certain 

 cells portrayed in black 

 at the end of the annual 

 ring, or, as it is com- 

 monly phrased, on the 

 face of the summer wood. 

 These elements are parenchymatous and constitute the only 

 representatives of this category of storage tissue in the wood of 

 Picea. Fig. 33 presents a longitudinal aspect of the face of the 

 summer wood in the same species. In order that a considerable 

 length may be shown, three successive segments of the plane of 

 section are depicted. In the median segment a row of short cells is 

 seen. These have obviously simple pits in their walls in relation 

 both to one another and to adjacent elements in the wood belonging 

 to a different category. In other words, the cells under considera- 

 tion are typical parenchymatous elements in longitudinal view. 

 To the left lies another and lower segment. At the top of this is 

 the continuation of one of the parenchymatous elements of the 

 first-described segment. As we follow the series of short cells 

 downward in the segment to the left the character of the elements 



FIG. 32. Detail of transverse section of the 

 vood of the root in Picea. 



