BETA VULGARIS 



2-75 



Senility is indicated by the formation of definitive callus on the 

 sieve plates; and, eventually, the tube is crushed. (Fig. 136, 77.) 



Fig. 136. Ontogeny of sieve tubes in leaf of sugar beet, i, young sieve tube with slime 

 bodies, plastids, and disintegrating nuclei; i and 8, younger; 3 and 9, older sieve tubes and 

 companion cells from secondary phloem; 4, young sieve tube and companion cell from pri- 

 mary phloem ; 5 and 6, portions of mature sieve tubes showing primary lamellae and second- 

 ary thickenings on sieve plate, slime accumulations, and plastids; 7 and 11, definitive callus 

 on sieve plates, plastids, and cytoplasmic network of old sieve tubes; 10, sieve tube and com- 

 panion cell in early stage of obliteration and two crushed cells; 11, crushed sieve tube and 

 companion cell; 13 and 14, two stages in obliteration; cai^csillusfcc, companion cell; nuc, 

 nucleus; 0^/, obliterated cells; /»/, plastid; j/, slime bodies; j^, sieve tube. (After Esau.) 



