276 



HOW CROPS GROW. 



and deep longitudinal rifts, and it gradually decays or 

 drops away exteriorly as the newer bark forms within. 



Cork is one form which the epidermal cells assume on 

 the stem of the cork oak, on the potato tuber, and many 

 other plants. 



Pith Hays. Those portions of the first-formed cell- 

 tissue which were interposed between the young and orig- 

 inally ununited wood-fibers remain, and connect the pith 



with the rind. In hard stems 

 they become flattened by the 

 pressure of the fibers, and are 

 readily seen in most kinds of wood 

 when split lengthwise. They are 

 especially conspicuous in the oak 

 and maple, and form what is com- 

 monly known as the silver-grain. 

 The botanist terms them pith-rays 

 or medullary rays. 



Fig. 51 exhibits a section of a 

 bit of wood of the Red Pine, 

 (Pi)ius picea,) magnified 200 di- 

 ameters. The section is made 

 tangential to the stem and length- 

 wise of the wood-cells, four of 

 which are in part represented, h ; 

 it cuts across the pith-rays, whose 

 cell-structure and position in the wood are seen at m, n. 



Cambium of Exoyens. The growing part^of the exog- 

 enous stem is thus found between the wood and the bark, 

 or rather between the fully formed wood and the mature 

 bark. There is, in fact, no definite limit where wood ceases 

 and bark begins, for they are connected by the cambial or 

 formative tissue, from which, on the one hand, wood-fibers, 

 and on the other, bast-fibers, or the tissues of the bark, 

 rapidly develope. In the cambium, likewise, the pith-rays 



Fte. 51. 



