iSKCOXDAUV GUOWl'll IX liOOTS 133 



tlie zone of root hairs. According to Ilayward (p. 51), in some 

 plants the adjacent cells of the endoderinis may contribute to the 

 formation of the lateral root primordinm. Arnold (li)40) has 

 shown that in the water hyacinth {Eichhornia crassipes) lateral 

 roots arise in the "innnature i^ericycle" and "at the forward end 

 of the region of elongation." Because of its internal origin, the 

 further develoi^ment of the lateral root involves its penetration 

 through the endodermis, cortex and epidermis of tlie mother root 

 to the outside. Just how this occurs is not entirely clear. The 

 suggestion has been made that the mechanical pressure exerted 

 by the emerging lateral root may also be accompanied by some 

 kind of chemical dissolution of the tissues interposed in its path. 

 In roots with three or more xylem plates, lateral root primordia 

 typically a])pear opposite each of the protoxylem points. Con- 

 sequently, ludess injuries or abnormalities occur, the lateral roots 

 tend to emerge in vertical rows which are equal in number to the 

 xylem groups. T>ut in diai-ch steles, lateral roots may appear 

 at each side of tlie two phloem groups. In this case there w^ould 

 be four vertical rows of lateral roots (cf. Esau, 1940, pp. 

 190-194). 



2. Secondarji (jroirth in roofs. The roots of many herbaceous 

 dicotyledons and of all woody plants exhibit secondary growth in 

 thickness. However, because of the radial, alternate arrangement 

 of the primary vascular tissues, the cambium first appears as sepa- 

 rate bands of periclinally-dividing cells which originate from 

 ])arenchyma cells internal to each phloem group. At these points 

 formation of secondary phloem outwardly, and secondly xylem 

 inwardly, occurs as in a typical stem. In woody plants, the orig- 

 inally separate strips of cambium finally become united laterally 

 as a result of tangential divisions in the pericycle external to 

 each xylem group. Thus at an early stage, the vascular cambium 

 in this type appears lobed in trans-section. Ultimately, by the 

 formation of secondary phloem and xylem external to the xylem 

 plates, the contour of the cambium becomes cylindrical. In roots 

 of this type, the primary xylem eventually becomes completely 

 surrounded by a cylinder of secondary xylem. In certain her- 

 haceous plants, in contrast, the cambium, at points opposite the 

 protoxylem points, forms broad parenchymatous rays so that a 



