410 WILLIS ; MORPHOLOGY OF THE PODOSTBMACB3Æ 
hardly any typical ” root character left to it. It is 
developed from the base of the hypocotyl, sometimes endo- 
genously but often exogenously, it increases by marginal 
instead of apical growth, is irregularly lobed, has a marginal 
“root-cap,” and performs the function of assimilation, in 
addition to bearing the floriferous shoots. 
As mentioned above in dealing with Hydrobryum, if we 
had only these plants before us, it would be difficult to know 
what to make of these thalli, but with the series of grada- 
tions before us, we must consider them as homologous Avith 
the thalli of Tristicha and the American Podostemons. 
These being unquestionably roots, thougli adventitious, in 
the ordinary morphological sense, there is no reason to 
refuse the term root to any of these thalli. On the other 
hand, though this is no doubt true, if we call these thalli 
roots we must use the latter expression in a very broad and 
A^ague sense, and as a classificatory rather than a descriptive 
term, much as we use the terms Dicotyledon, Fungus, 
Flower. The term root has so much more specialized and 
functional a meaning"'^ than the terms stem or leaf in 
botanical work, that it is probably better not to apply it 
definitely to the roots of most of these Podostemaceæ, but 
to describe the organs in this family as thalli, leaving for 
further specification in each case whether the thalius is of 
“shoot” or “root” nature. None of the ordinary defini- 
, tions of roots can be made to include the thalli of such plants 
as Hydrobryum Avithout stretching them so much that it is 
difficult to exclude stem forms. 
This of course is assuming that these thalli are phylogene- 
tically descended from undoubted roots, i,e.^ t])at some of 
their ancestors Avere provided with roots developed “ adven- 
titiously” at the base of the hypocotyl. There is eA^ery 
reason to suppose that such Avas the case, for Ave know this 
to^be an extremely common feature in many plants, and 
especially in those Avhich are unable for physical reasons to 
* The reasons for this have often been pointed out, e.g., by Sachs and 
Goebel. 
