ISOETES JAPOmCA, A. BE. 



349 



t 



2. Frimary I'hloem. 



The primary pHoem of tlie rliizopliore-stele consists of sieve-tubes whicli are essentially 

 similar to those of tlie primary phloem of the stem ; they are accompanied by unmodified 

 cells of the ' parenchymatous mantle/ which may possibly constitute the phloem- 

 parenchyma. 



3. Frimary Cortex. 



The primary cortex of the rhizophore region needs no further description. 



Steuctuee of the Secondaey Tissues op the Ehizophoee 



1. The Cambium. 



In the rhizopliore cambial activity starts very early, the cambium arising from the 

 peripheral cells of the 'parenchymatous mantle' just below the apical meristem 

 (text-figs. 4 & 5). 



2. Secondary Fliloem. 



The tissue cut off internally by the cambium consists of parenchyma and sieve-tubes 

 which are exactly similar to those of the secondary phloem of the stem. 



3. Secondary Cortex. 



The greater part of the caudex of Isoetes consists of secondary cortex, derived partly 

 from the cambium of the stem and partly from the cambium of the rhizophore. 



The secondary cortex of the rhizophore consists of large spherical parenchymatous 

 cells containing starch and proteid granules. These cells are cut off externally by the 

 cambium, which completely surrounds all three lobes of the rhizophore-stele apart from 

 the region of the apical meristem (PI. 40. fig. 80, and text-figs. 4, 5). The lobing of 

 the caudex depends entirely upon the distribution of the cambium, since the three 

 main furrows always coincide with the three narrow zones of primary meristem, from 

 which, as stated above, relatively very few cortical cells are differentiated. This 

 accounts for the almost spherical outline of the caudex of the very young sporophyte, 

 in which the formation of secondary cortical tissue has not had time to proceed 

 ^Qvj far. 



Owing to the centripetal development of the secondary cortical tissues, the diameter 

 of the caudex rapidly increases ; but nevertheless a very considerable increase in height 

 as compared with the increase in diameter is noticeable in old specimens (PL 33. 



figs. 4 7). This result is brought about by the constant decortication of the older 



{i. e. most peripheral) region of the caudex. This process of decortication takes place 

 in the following manner. Certain irregular patches of cortical cells situated at or near 



the periphery of the caudex are stretched by the mechanical pull exerted by the 

 rapidly increasing radial growth of the secondary cortex (text-fig. 6). The walls of the 

 cells bordering on these ' islands ' or ' bays ' of stretched parenchyma gradually become 



