RADIAL, AND CO^X'EXTRIC BUNDLES 57 



nearer the center being gradually larger. The various 

 xylem strands may meet in the center in one large vessel 

 or in a mass of tracheary elements, or the center may 

 consist of parenchyma, or of sclerenchyma, or even of 

 fibrous tissue. Midway between the xylem strands, 

 and like them bordering upon the pericycle are smaller 

 or larger phloem masses, consisting 

 mainly of large sieve tubes, and small 

 companion cells, and other parenchyma 

 cells. The tissue between the phloem ^^^ 24— Half of 

 and xylem strands may be parenchyma | radial vascular 

 or in part fibrous or sclerenchyma. 



83. Lateral roots arise by the conversion of portions 

 of the pericycle into active meristem cells which soon 

 become arranged in definite layers, as in the growing 

 root tip. This rootlet forces its way out through the 

 cortex until it reaches the outside. The plerome part 

 becomes the vascular bundle whose tracheary and sieve 

 elements are connected respectively with the xylem and 

 phloem strands of the main bundle. 



84. The concentric type of bundles is found mainly 



in the stems and leaves of Ferns and 

 Fern Allies. In these plants the stem 

 usually possesses several vascular bun- 

 dles, which may be variously located 

 Fio. 25.— Concentric ^ud of different shapes and cross- 



vascular bundle. '■ 



sections. They branch more or less 

 frequently and in some cases anastomose very freely. 

 Some of the bundles pass out from the stem into the 

 leaves, there to branch again to form the veins. In 

 general, the bundle consists of a plate of xylem, sur- 

 rounded on all sides or on all except the edges of the 

 plate, by large sieve tubes and small parenchyma cells. 

 Around these are often one or more layers of starch-bear- 



