ISOETES JAPONICA, A. BE. 



359 



I. japonica tlie pbyllotaxis is -,% ; but we were unable to determine the complicated 

 pliyllotaxis of older plants with any degree of accuracy. On tlie other hand, 

 Hofmeister (i6, p. 536) points out tbat the leaves of young plants of I. lactcstHs, a 

 bi-lobed speeies, are always at first arranged in a distichous manner, the distichous 



ingement gradually passing over into a spiral of increasing complexity 

 until finally an /r phyllotaxis is attained. 



3 



Development of the Leaves. 



The primordia of the leaves arise from certain superficial cells of the primary cortex 

 situated around the base of the apical protuberance (PI. 39. figs. 65-70). As a result 

 of the luxuriant growth of these superficial cells of the primary cortical tissue, the 

 primordia can very soon be identified as smaller or larger, flattened, more or less ovate, 

 or crescent-shaped papillae. Subsequent growth of the leaf, as Farmer (14, P- 46) has 

 already described, is intercalary, the meristematic tissue being for the most part 

 restricted to a zone (of varying thickness) at or near the base of the young leaf. 

 Ultimately, however, this meristematic zone becomes localized in a region immediately 

 above the insertion of the liffule 



s 



the stem 



It must be remembered, however, that considerable elongation results from the 

 further growth, both in length and in diameter, of the individual cells of the leaf 



{cf. Wilson-Smith, 30, pp. 229-230). 



That part of the procambial strand of the foliar bundle which traverses the primary 

 cortex, is differentiated at a very early stage in the development of the leaf. It 

 originates by the division of certain cells of the primary cortex, which retain their 

 meristematic character for a considerable period. A strand of small cells, easily dis- 



^uishable by their relatively large nuclei, is produced in an upward and outward 

 direction (PI. 39. fig. 70). The upper extremity of the strand extends to the base of the 

 young leaf. Connection with the primary xylem and phloem tissues of 

 is established by the downward prolongation of the procambial strand, the tissues 

 of which are differentiated from the ' parenchymatous mantle.' 



The difi^erentiation of the procambial strand of the leaf immediately precedes the 

 differentiation of the cambium of the stem from the outermost cells of the ' parenchy- 

 matous mantle'; hence it is evident that the developing leaf-trace is not interrupted 

 either by the cambium itself or by the products of its activity. We agree with Farmer 

 (14, p. 40) that Hegelmaier's statement that the cells of the cambium contribute to the 

 formation of the leaf-trace is incorrect, for. as Farmer {I.e.) remarks, the particular 

 direction of the division of the cells which take part in the formation of the leaf-trace 

 is determined before the cells in their vicinity assume the freshly active meristematic 



condition. 



Further differentiation of the procambial strand of the leaf takes place centrifugally, 



the cells of the basal part of the strand being the first to become converted into 



tracheids, sieve-tubes, etc. Tliis process gradually extends upwards and 



towards the base of the leaf until finally the lamina is reached. 



ards 



SECOND SERIES. — BOTANY, VOL. VIII. 



3 



