398 PRIMARV ARRANGEMENT OF TISSUES. 



Janczewski's //ii'rd type (Fagopyrum, Raplianus, Helianthus) stands near the second, 

 inasmuch as the entire ruiliment of the lateral root arises from the pericambium, with 

 little or no participation of the endodermal layer. It is distinguished from the second 

 t)-pe by the regularity of the initial divisions. The cells of the segment of the peri- 

 cambial layer concerned, become elongated in a radial direction, and divide once 

 tangentially. The inner of the two layers derived from the latter division forms the 

 commencement of the new plerome bundle ; the outer is divided again tangentially 

 into an outermost layer, which is the calyptrogen, and an inner, or middle layer, which 

 forms the commencement of the entire corte;;. In Helianthus the endodermal layer 

 of the mother root forms a many-layered external covering to the root-cap, over the 

 apex of the rudiment of the lateral root ; in Fagopyrum it only grows to form a 

 single-layered envelope of the latter ; and in Raphanus it remains quite passive. 



In the fourth type, which embraces the Cucurbitaceae and Papilionaceae 

 mentioned at p. 12, the rudiment of the lateral root is derived from the con\mon 

 growth of the portion of the pericambium in question, of the endodermal layer, and 

 of the 1-2 cortical layers adjoining the latter externally. The former constitutes the 

 plerome bundle, the latter conjointly form the surrounding portions of the rudimentary 

 root, under the apex of which the common initial zone is differentiated at a relatively 

 late period. 



The roots of Pinus constitute ihefi/ih type. The whole rudiment of the lateral 

 root, which soon assumes the differentiation described at p. 13, proceeds from the 

 many-layered pericambium ; the endodermis and the layers adjoining it externally 

 remain passive. In the Cycadeae, in Taxus, and in Sequoia, on the other hand, the 

 latter, according to Reinke and Strasburger, take part to a small extent in the 

 formation of the most peripheral layers of the rudimentary root ^ 



In opposition to the Phanerogams the lateral rudimentary roots, in the case of the 

 monopodially branched roots of all investigated Ferns and 3IarsiliacecB^, take their 

 origin from the endodermal layer which surrounds the pericambium ; in the Equiseia 

 they arise from the layer lying within the endodermis (comp. p. 351). In many 

 Cyatheaceae and in Marsilia the longitudinal rows of endodermal cells, which may be 

 shortly described as rhizogenetic rows, and which, as corresponding to the places 

 of origin of the lateral roots, face the xylem-plates, are distinguished by their 

 wider and shorter cells from the other rows of the same layer; frequently those 

 rows of the next outer cortical layer, which lie in front of the rhizogenetic rows, 

 show similar relations of size, and also have their walls less thickened than the other 

 rows belonging to the same layer. The layers however which lie outside the 

 endodermal layer take no active part in the origination of lateral roots. On the 

 contrary, every lateral root proceeds from a cell belonging to the rhizogenetic layer, 

 which directly, or after a few irregular preliminary divisions, assumes the character- 

 istics of the apical cell of the root, as described at p. 18. The pericambium of the 

 mother-root is only concerned in the formation of lateral roots, in so far as the 

 connecting piece between the vascular bundles of the two orders is formed in it. 



As already indicated, the place of origin of a lateral root in the Ferns and 

 Equisetum is always a circumscribed portion of tissue lying in front of a xylem-plate. 



' Strasburger, Die Coniferen, &c., p. 348. * Nageli und Leitgeb, I.e. p. 88. 



