RADIAL, AND CONCENTRIC 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 Le 
mainly of large sieve tubes, and small AG Sab 
companion cells, and other parenchyma > ES 
cells. The tissue between the phloem 27-3. har ot 
and xylem strands may be parenchyma ¢ t3dial 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 
Eis = 1:7 in the stems and leaves of Ferns and 
Ef cerpoiei ae} Fern Allies. In these plants the stem 
Oaeeiiey usually possesses several vascular bun- 
dles, which may be variously located 
Fia. 25.Concentric and of different shapes and cross- 
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- 
