CUCURBITA 



615 



pointed, conical hairs; short-stalked, and long-stalked, glandular 

 ones. 



The bundles are bicollateral but not as large as those in the hypo- 

 cotyl, and there is less cambial activity. The size of the secondary 

 xylem vessels is about the same as in the root, and the sieve tubes 

 may be larger, reaching a diameter of 0.088 mm. There is fre- 

 quently a well-defined inner cam- 

 bium in the larger bundles which 

 produces some phloem and pa- 

 renchyma, but a lateral perixylary 

 cambium is usually lacking. Iso- 

 lated strands of phloem occur in 

 the parenchymatous tissues of 

 the pericylic rays and medulla; 

 and, to a limited degree, in the 

 parenchyma between the epider- 

 mis and the sclerenchymatous 

 ring. Commissural or connective 

 sieve tubes serve to join all the 

 phloem elements into a continu- 

 ous system. 



Crafts (4) has described the 

 sieve tubes as being of two types: 

 (i) the central tubes of the phloem 

 proper which consist of short, 

 broad elements; and (i) the pe- 

 ripheral tubes, including commis- 

 sural, entocyclic, and ectocyclic 

 phloem, which vary greatly in 



cav 



Fig. 311 . Transection of sector of mature 

 stem : ca, cambium ; cav, medullary cavity ; 

 «/, collenchyma; f/;, epidermis ; iph, inner 

 phloem; o ph, outer phloem; par, paren- 



length, but tend to be of uniform ^hy'"^; /"• P"^; sd, sderenchyma ; xy I, 

 ^ primary xylem ; xj i, secondary xylem. 



diameter. Slime droplets occur 



in the young sieve tubes, and protoplasmic connections, which 

 are continuous with the parietal cytoplasm, traverse the sieve 

 plates connecting successive elements in mature sieve tubes. 

 Callus develops around each cytoplasmic strand that passes through 

 the sieve plate, and the callus cylinders increase in diameter until 

 they fuse laterally to form a perforated plate which increases in 

 thickness. The callus may become definitive, and slime plugs 

 occur under certain conditions. In the peripheral phloem the 

 appearance of the definitive callus takes place late in the ontogeny 



