474 SUMMARY OF CURRENT RESEARCHES RELATING TO 



dichotomous, but by one-sided growth becomes monopodial. In the 

 rhizome, the main branch is shorter than the frond branch ; in the 

 frond the main branch grows quicker than the lateral. The bundle of 

 the rhizome consists of inclosed funnel-shaped tubes fitting into one 

 another and separated by ground tissue. In the lateral branches of the 

 rhizome, the outer tubes form meshes. In transverse section, the 

 rhizome exhibits one, two or more concentric bundle-rings separated by 

 ground tissue or transitional tissue. Primary tracheids occur on the 

 periphery of the outer xylem ring. Weak rhizomes possess a central 

 cylindric immature bundle, with central xylem surrounded by phloem 

 and sheath, and growing by central additions. The fronds arising from 

 the rhizome possess in their stalks a number of single parallel bundles. 

 In weak fronds are found only a double bundle or two single ones. 

 These bundles increase to ten or more and anastomose ; in transverse 

 section they appear arranged in three rows. Epidermis and cortex have 

 normal structure. The lower leaves of the rhizome differ much in struc- 

 ture from the highly differentiated aerial leaves. The rhizophores are 

 produced only at the points of branching of the rhizome and remain 

 short and simple. They arise exogenously from prismatic initials, and 

 their brief meristematic apical growth is soon changed to intercalary, 

 and during the latter mode of growth the root is differentiated. The 

 root at its apex shows a sharp differentiation into three layers of 

 meristem. There is the independent root-cap with the calyptrogen ; the 

 root covered by the dermatogen ; and the periblem and plerome. The 

 root, like the stem, is dichotomously branched, and, owing to unequal 

 development, becomes monopodial. The roots arise only from the 

 rhizophore and are produced endogenously ; their rudiments exhibited 

 a well-marked dermatogen. The structure of root and rhizophore is 

 normal, and these two are distinguished by the structure of their 

 epidermis. No endophyte was found in the rhizoids. 



Anatomy of Isoetes.* — -A. (4. Stokey gives an account of the 

 anatomy of Isoetes, based upon American species. In a summary of ber 

 results, she says : 1. The vascular axis is a non-medullated monostele, 

 composed of tracheids and parenchyma ; there is no differentiation into 

 protoxylem and metaxylem. 2. No primary phloem in stem ; it occurs 

 only in leaf-traces and root-bundles. 3. Cambium produces cortex out- 

 side and secondary xylem inside, and forms no phloem. 4. Secondary 

 xylem consists of various combinations of the following : Spiral or 

 annular tracheids ; immature tracheids (either slightly lignified or un- 

 lignified, with irregular rings or spirals, or with pits, nucleate or 

 enucleate) ; parenchyma with more or less protoplasm. 5. Zonation in 

 secondary xylem depends on the species. 6. Root-bundles are collateral 

 and monarch ; protoxylem is endarch. 7. Leaf -traces are collateral, 

 becoming concentric in upper part of leaf. The xylem is much reduced 

 above the sporangium. The sieve-plates are transverse. 8. Near the 

 vascular axis the leaf -trace does not show differentiation into protoxylem 

 and metaxylem. In the outer cortex and near the sporangium it is 

 usually exarch. 



* Bot. Gaz., xlvii. (1909) pp. 311-35 (3 pis.). 



