vas bu 



-- -med cav 



614 THE STRUCTURE OF ECONOMIC PLANTS 



ray regions. All of these phloem types are interconnected by 

 strands of connective phloem, so that this diverse and complex 

 system may be regarded as having continuity although some of 

 the elements end blindly in parenchymatous tissue at various points 



in the axis. 



The Stem. — The vascular bundles of the stem are arranged in 

 two rings. The smaller ones of the outer ring are located at the 

 angles of the stem, and the larger bundles of the inner ring are 

 alternate with those of the outer. (Fig. 3x0.) The basic number 



of bundles is ten, each cycle 

 consisting of five, although 

 occasionally additional 

 smaller ones may be pres- 

 ent. All the bundles are 

 common, and the bi-cyclic 

 character of the dictyostele 

 is related to the mode of 

 insertion and downward di- 

 vergence of the traces of the 



Fig. 1,7.0. Diagrammatic transection of stem ^ , , , 



showing distribution of bundles : «/, coUenchyma ; leaf. Each vaSCukr Strand 



ep, epidermis ; med cav, medullary cavity ; par, paren- eXtends downwatd thrOUgh 



chyma; ../, sclerenchyma ; .^x ^ vascular bundles. ^^^ ^^^^ ^^^ ^^ average of 



two internodes before anastomosing with another bundle. Within 

 the inner ring, the pith becomes disintegrated, and a large irregu- 

 larly shaped cavity is formed. 



Outside the outer ring of bundles and continuous with the 

 medulla is a broad zone of parenchymatous tissue. This is limited 

 externally by an uninterrupted band of sclerenchyma five or more 

 cell layers in width; but the continuity may be broken in old 

 stems by secondary growth. (Fig. 311.) The parenchyma and the 

 adjacent fibers together constitute the pericyclic zone. There is 

 a narrow band of chlorenchyma outside the sclerenchymatous 

 ring; and between this and the epidermis is a discontinuous band 

 of collenchyma several cells in width which is interrupted by the 

 chlorenchyma at regular intervals. The epidermis is regular with 

 a thin, smooth cuticle, and stomata occur, where it is subtended 

 by chlorenchyma, in a frequency of about xo per sq. mm. Numer- 

 ous rigid multicellular hairs are produced on somewhat raised 

 bases; and of the four types of hairs occurring in the Cucurbi- 

 taceae, three are present on the stems of C. Pepo. These are sharp- 



