32] CHAPTER II. THE TISSUES. 119 



times termed the trannfusion-tisstte] namely, cells with unlignified and 

 unpitted walls, distinguished by their abundant protoplasmic and proteid 

 contents ; tracheidal cells with slightly lignified walls and bordered pits, 

 without protoplasmic contents ; the former may be regarded as an ex- 

 tension of the sieve-tissue of the bundle, the latter-as an extension of the 

 tracheal tissue. 



The Pericycle is altogether wanting in a few cases only ; it is 

 absent when the endodermis consists of two layers (see p. 115) ; it 

 is also absent from the slender roots and stems of some water- 

 plants. 



It is usually a continuous membrane; but in some cases it is 

 interrupted by projections of the vascular tissue (e.g. by the- 

 xylem-bundles in the root of some Graminese and Cj^peraceae). It 

 may consjst throughout of a single layer of cells (e.g. roots of most 

 Angiosperms [Fig. 96] and of some Vascular Cryptogams) ; or of 

 more than one layer throughout (roots of some Dicotyledons, e.g. 

 Vine, and of Gymnosperms generally ; commonly in the stem and 

 leaf-stalk) ; or in part of one layer and part of more than one (e.g. 

 root of some Perns and Leguminosse). 



The pej-icycle-jnay be hamogeneous or heterogeneous ; that is, it 

 may consist of the same kind of tissue throughout, or of several 

 kinds of tissue. The typical homogeneous pericycle consists of 

 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 Monocot} r ledons. 



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, Hyperi- 

 cacese) ; ^t never contains fibres. 



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

 is generall}'- heterogeneous, owing principally to the differentiation 

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

 hausia foetida, Sonchus oleraceus}, or into fibres (Fig. 97) which 

 are generally sclerotic, but not in all cases (e.g. Apocynacese, Con- 

 volvulacese, Flax) ; or it may be heterogeneous in consequence of 

 the presence of secretory ducts (e.g. Hypericum, some Umbelli- 

 ferse) ; or, in consequence of the presence of both secretory ducts 

 and of fibres (e.g. Ligulifloral and Tubulifloral Composites). 



The Pith (or medulla) consists, typically, of parenchymatous 



