116 THE STEM 



(c) Fibers, which are sununvhat irregular in shape and are 

 usually much smaller than either the vessels or tracheids and are 

 usually provided with thicker walls. 



(d) Scattered among the vessels, tracheids and fibers, occur 

 the wood parenchyma cells which are small in size, isodiametric 

 in form, and possess a definite protoplast. It should be realized 

 that wood parenchyma cells occur in "vertical chains" or rows 

 and function primarily in the storage of certain carbohydrates, 

 particularly starch; the function of wood parenchyma in "assist- 

 ing" the trans-location of substances in the xylem is imperfectly 

 understood. In Tilia, the wood-parenchyma has no definite dis- 

 tribution in the xylem and is hence termed "diffuse." 



Extending radially through the secondary xylem are the rays 

 which are of two types, viz. : 



{a) Large rays, which represent the xylem "extension" of 

 the large si^ecialized dilated rays of the phloem discussed pre- 

 viously. In the second annual ring, these large rays may be 

 two or three cells in width but, at least in the inner portion of 

 the first annual ring, they become a single cell in width and 

 finally terminate directly in the pith. Such rays have been termed 

 "primary medullary rays." 



{h) Small rays, one cell in width which are the xylem ex- 

 tension of the rays extending through the "banded sectors" of 

 the phloem. These rays have been termed "secondary medullary 

 rays" but in some instances at least have no direct connection 

 with the pith. 



The pri))i<try xylem occurs next to the pith and is completely 

 surrounded externally by the cylinder of secondar}- xylem. It 

 is naturally very difficult to distinguish between the metaxylcm 

 and the first-formed elements of tlie secondary xylem in a trans- 

 verse section of the stem of Tilia. The protoxylem, however, 

 is quite distinct and appears as solitary or grouped tliick-walled 

 cells which are imbedded among the small parenchyma cells of 

 the primary xylem at the periphery of the pith. The pith is a 

 solid rod of tissue occupying the center of the stem. In contrast 

 to tlie condition in the geranium stem, the pith of Tilia is not 

 homogeneous but shows a certain amount of tissue specialization 

 as follows : 



