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ADA HAYDEN 
similar root habits show little storage area but have prominent tracheae, 
which would indicate that they derive their moisture from a lower water 
table than do their short-rooted neighbors. Liatris shows few tracheae 
but much storage tissue. Ceanothus has also a deep, branched tap root. 
The exterior of the cortex of the roots is corky, leathery, fibrous-sheathed, 
or flaky-deciduous, while the subterranean stems have hard, scaly fibrous 
or straw sheaths which probably have the ability to hold water by capil- 
larity as well as to prevent evaporation from the inner parts. These scaly 
coverings of thickened subterranean organs, especially of rhizomes, are 
prominent in both uplands and alluvial basins. Since conditions of drought 
are likely to occur frequently and especially in protracted periods during 
the latter part of the summer, these features are undoubtedly useful as a 
protection against desiccation. The swamp plants must be able to tolerate 
not only moisture to the point of saturation but drought as well. The 
roots of the upland plants may be regarded as of the intensive type, those 
of the lowlands as of the extensive type. 
Minute Anatomy 
The anatomical descriptions of the stems and roots studied have been 
made under the heads (a) Primary cortex, and (b) Stele, which portions of 
the stem and root are homologous. These structures show certain prev- 
alent types which may be distinguished in the angiosperms as follows: 
Monocotyledons — 
Stem: concentric or collateral, endarch bundles. 
Root: radial, exarch bundles. 
Dicotyledons — 
Stem: concentric, endarch bundles. 
Root: radial, exarch bundles. 
There is great diversity of structure shown by the representatives of 
different families and individuals thereof, whose morphological and physio- 
logical variations are discussed by De Bary, Solereder, Haberlandt, Stevens, 
Coulter, Barnes, Cowles, and others. Exceptions seem more abundant 
than cases of conformity to rules at the present status of correlation. Trans- 
formations from the original types take place with the secondary thicken- 
ing process, in numerous instances to such an extent that the original type 
structures are hardly recognizable. Systematic relations have not been 
conclusively worked out, yet every species has in some degree established 
its economy of water relations, the indicators of which, in so far as experi- 
ment has proceeded, are shown to be primarily (a) parenchyma (storage 
and aerenchyma), (b) mechanical tissue, and {c) conductive tissue. The 
prominence of the latter two seems to indicate xerophytism and that of the 
first the reverse condition. The relative proportion of these tissues in each 
individual stem or root has been used as a means of indicating its degree of 
xerophytism in this study. 
