348 MESSES. CYEIL WEST AXD H. TAKEDA ON 



stem-stele in several important details. Eor instance, the apical nieristem of the 

 rhizophore is distributed over a much larger area, since it extends along the whole 

 length of the curved lower edge of each of the three lobes of the stele as indicated in 

 text-figs. 1, 4, 5. 



However, it must be remembered that this primary meristem is confined to a very 

 narrow strip of tissue and is completely absent from the lateral surfaces of the stelar 

 lobes. This is clearly shown in text-figs. 4 and 5, which indicate in a diagrammatic 

 manner the arrangement and distribution of the various structures met with in transverse 

 sections through one of the lobes of the rhizophore-stele. PI. 40. fig. 77 is reproduced 

 from an untouched photo-micrograph of a transverse section through a similar lobe, cut 

 near its apex. In this figure, only those tissues which are found along the lower margin 

 of the lobe are shown. Prom the central region of this primary meristem the tracheids 

 and parenchyma of the xylem are slowly differentiated (PL 39. fig. 63 ; PL 40. figs. 75, 

 77, and text-figs. 4, 5; also cf. Hofmeister, i6, Taf. 52. fig. 6). This results in the 

 continual addition of xylem elements to tlie primary stele of the rhizophore. Prom 

 the more peripheral regions of this meristem the 'parenchymatous mantle,' primary 

 phloem, and cambium of the rhizophore-stele are produced (text-figs. 4, 5). 



The primary meristematic tissue extends to a point slightly beyond the apex of 

 the stelar lobe, so that the ^parenchymatous mantle' and primary phloem are found 

 along the upper surface as well as on both flanks of the lobe (text-figs. 1, 4, 5). 

 Consequently, the actual growth of the primary tissues of the rhizophore-stele is 

 confined to three radiating lines («, 6, c, in text-fig. 10) separated from one another by 

 an angle of 120° and inclined at an angle of approximately 80° to the vertical axis of the 

 ])lant (text-figs. 1, 11). Hence, there can be no lateral growth of the primary tissues 

 of the stelar lobes of the rhizophore, and consequently no lateral increase in dimensions 

 apart from that brought about by the extension of tissues already formed. However, 

 the primary tissues of the rhizophore-stele are subjected to a pulling strain by the 

 out-going root-traces (PL 40. fig. 80, and text-figs. 4, 5). 



Tlie primary meristem of the rhizophore is situated not at the actual periphery of the 

 caudex, as in the stem apex, but is separated from the exterior by several regularly 

 arranged layers of parenchymatous cells (PL 38. fig. 63 ; PL 40. figs. 75, 77), which 



contain numerous large starch-grains. These cells (=:primary cortex of the rhizophore) 

 form a structure in many respects analogous to a * root-cap.* 



Iso'etes lacustris was examined for comparison with the above, from which it differs 

 only in the possession of two, instead of three, lobes to the rhizophore-stele. 



Stuucture of the Primary Tissues oe the Ehizophore. 



1. Primary Xylem. 



The tracheids of the rhizophore-stele diff'er both in shape and size from those of the 

 stem-stele. The former are not oval in shape but are considerably elongated (PL 38. 

 fig. 63 ; PL 40. figs. 75, 77) ; they are rather smaller than the tracheids of the stem-stele 

 and are spirally or reticulately thickened. 



