168 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[32. 



thin-walled parenchymatous cells, with protoplasmic contents, 

 which are capable of becoming merismatic. In some cases the 

 primarily thin- walled cells eventually become sclerotic, either 

 throughout the whole pericycle, or in certain parts only ; this 

 commonly occurs in the roots of Monocotyledons. 



Generally speaking, the pericycle of the root is homogeneous ; 

 when it is heterogeneous, it is so in consequence of the presence 

 of glandular tissue (secretory ducts) (e.g. Umbelliferae, Araliaceae, 

 Pittosporese, Hypericaceee) ; it never contains fibres. 



FIG. 131. Tranverse section of a root of AHium Cepa, showing the central stele without 

 pith: c cortex; e endodermis; p pericycle; sp and o, spiral and annular vessels 

 (protoxylem) ; sc pitted wood. vessels of the primary wood ; v bast-bundle ; the six wood- 

 bundles alternate with the six bast-bundles ; the wood-bundles develope centripetally 

 and meet, so that no pith is formed. (After Strasburger : x 240.) 



The pericycle of the stem and of the leaf-stalk, on the contrary, 

 is generally heterogeneous, owing principally to the differentiation 

 of a portion of it into collenchyma (e.g. some Composites, Bark- 

 hausia fostida, Sonchus oleraceus), or into fibres which are gener- 

 ally sclerotic, but not in all cases (e.g. Apocynacese, Asclepiadaceee, 

 Convolvulaceae, Flax) ; or it may be heterogeneous in consequence 

 of the presence of secretory ducts (e.g. Hypericum, Pittosporum, 



