34] CHAPTER II. THE TISSUES. 135 



the pericycle is wanting opposite to the xylem-buudles, the lateral 

 roots are developed, not opposite to the xylem-bundles, but opposite 

 to the phloem-bundles. Again, when there are only two xylem- 

 bundles in the parent root, four rows of lateral roots are produced, 

 each root being developed on one side of a xylem-bundle of the 

 parent root : a similar displacement occurs in Umbelliferse, 

 Araliacese and Pittosporacese, where the pericycle is interrupted 

 opposite to each xylem-bundle by an oil-duct (see p. 119). 



In most Vascular Cryptogams (except Lycopodium and Isoetes, 

 where secondary roots are produced only by dichotomy), the apical 

 cell of a secondary root is formed from one of a row of large 

 endodermal cells, the rhizogentc cells, lying just externally to 

 each xylem-bundle of the parent root. In Equisetum, where the 

 endodermis consists of two layers (see p. 115), the secondary roots 

 are developed from cells belonging exclusively to the inner layer, 

 which are adjacent to the xylem-bundles. 



It will be understood that, in order to reach the surface, the lateral 

 secondary roots must penetrate the external tissues of the parent root. 

 This is nq^effected by purely mechanical means, but by chemical action, 

 leading to solution and absorption, exerted on the tissues, either by the 

 rootlet itself, or, more commonly, by a digestive sac which invests the root- 

 let, and is formed in Phanerogams by the growth and division of the cells 

 of the endodermis (and sometimes one or two layers of cortical cells), in 

 Vascular Cryptogams, by the growth and division of one or more of inner 

 layers of cortical cells just external to the endodermis of the parent root. 



D. Development of Hairs. These structures are m_all cases 

 developed from the superficial cells of the parent member, that is, 

 from dermatogen-cells in those parts in which this layer is differ- 

 entiated ; in the great majority of cases each hair arises from a 

 single superficial cell. Hairs are generally developed in acropetal 

 succession, but considerable irregularity is not uncommon, and 

 they are frequently developed on members in which the tissues 

 have already acquired their permanent characters (see p. 46). 



E. Development of Emergences. When exogenous they are 

 developed from the superficial and from one or more of the sub- 

 jacent layers of tissue of the parent member, that is, from the 

 dermatogen and periblem of those members in which this differen- 

 tiation of the primary meristem obtains. When they are endo- 

 genous (e.g. haustoria of Cuscuta, see p. 49), they are developed 

 exclusively from the periblem. 



F. Development of Reproductive Organs. The question as to 



