370 MESSES. CTEIL WEST AND H. TAKEDA ON 



secondary tissues of this genus bear no resemblance \^'h.atever to those of any species of 



Lepidodendron or Sigillaria hitherto examined. 



The presence of a distinct root-bearing organ does not necessarily indicate any affinity 

 with the recent or fossil Lycopods which possess such a structure (e. g. Selaginella, 



Stigmaricty I^leuromeia, etc.). 



On the other hand, the anatomy of these structures is quite unlike that of the rhizo- 



phore of Isoetes, to which even the four-lobed root-bearing basal region of the caudex of 



Fleuromeia bears only a superficial resemblance. Whereas in the latter the lobes of the 



stele are situated in the centre of the caudex lobes (31, Taf. 8. fig. 6j, in the former they 



alternate with the lobes of the caudex and correspond in position to the main furrows. 



It is not unlikely that a distinct root-bearing organ has been developed along two 



three lines of descent, in which case the possession of a rhizophore is of little or no 

 value in determining the phylogenetic position of a plant. 



Unfortunately, the geological history of Isoetes is exceedingly meagre, a fact which 

 may be correlated witli the almost complete absence of hard tissues (apart from the ripe 



spores) from the sporophyte of this genus. 



We are accordingly led to the conclusion that Isoetes belongs to a separate Class of 

 Vascular Cryptogams, the Isoetales, co-ordinate with the Filicales, Lycopodiales, 

 Psilotales, Equisetales, and Sphenophyllales. 



Summary. 



The trilobed caudex of Isoetes japonic a consists of two distinct structures, viz. Stem 

 and MliizopJiore, to which the leaves and roots are respectively attached ; but owing to 

 the stunted growth of the plant, all external morphological ditferentiation between the 



two organs has been completely lost. 



Stem. — The stem-apex has the form of a conical mass of tissue situated at the base of 

 the funnel-shaped depression in the cortex. In this protuberance no definite apical cell 



can be distinguished. 



Primary xylem, phloem, and cortex are differentiated from the primary meristem of 



the stem. 



The cauline primary vascular axis is a non-medullated monostele. Primary phloem, 

 in which true sieve-tubes occur, surrounds the central xylem-core. 



An endophytic mycorrhiza is found in the peripheral cells of the primary cortex. 



The cambium, which arises very early from the outermost layer of the pie 



cuts off secondary cortex externally and secondary phloem internally. 



Sieve-tubes with sieve-areas of the typical cryptogamic type occur both in the primary 

 and in the secondary phloem, and are continuous with those of the leaf-traces. 



No secondary xylem is formed in Isoetes japonica. 



BUzophore. — The roots, the vascular bundles of which are collateral with usually 

 endarch protoxylem, are arranged in acropetal series upon a distinct root-bearing organ, 

 the rhizophore, ^vhich in this genus must be regarded as an organ sui generis. 



The primary growth of the rhizophore proceeds from a primary meristem situated 

 along three radiating lines which correspond to the main fissures in the caudex. 



