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. Umbelliferee, Araliaceae, 

 Pittosporacese, Hypericaceee) ; it never contains fibres. 



FIG. 131. Transverse section of a root of Allium Cepa, showing the central stele without 

 pith: c cortex; e endodermis ; p pericycle; sp+a, spiral and annular vessels (protoxylem); 

 c 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 BO 

 piih 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 Composite, Bark- 

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

 ally sclerotic, but not in all cases (e.g. Apocynaceae, Asclepiadacese, 

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

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



