488 REV. GEOUGE HENSLOTV ON A THEORETICAL 



subsequently became terrestrial, one would on a 'priori grounds 

 imagine that there would still be many aquatic endogens which 

 have not yet taken permanently to a terrestrial habit. Such is 

 precisely the case ; for of seventy-four British orders of exogens, 

 live only are decidedly aquatic, viz., Nymphceacece, Elatindcece, 

 llaloragece, Lentibularinece, and Ceratophyllacece : while aquatic 

 species are found in several other orders. 



On the other hand, there are fifteen British endogenous orders, 

 and of these eight are decidedly aquatic, viz., Ilydrocharidece, 

 Juncacea, Eriocaulinece, Typhacece, Lemnacece, Alismacece, Naia- 

 dacece, and Cyperacece ; while other orders have occasionally 

 aquatic or marsh plants. Hence exogenous aquatic British 

 orders are nearly in the proportion of 7 per cent., whereas in 

 endogens they reach 53 per cent. 



If we take a wider survey, as, e. g., of all the orders given in 

 Le Maout and Decaisne's ' Analytical Botany,' we find that out of 

 221 exogenous orders, 9 may be regarded as aquatic, or 4 per cent. ; 

 and of 55 orders of endogens, 18 are aquatic, or nearly 33 per cent. 



Such marked contrasts show that there is some decidedly 

 important connection between an aquatic habit and endogenous 

 structures ; for since exogens are more than four times as 

 numerous as endogens, the chances in favour of a preponderance 

 of exogens occurring in rivers, lakes, ponds, &c, is much greater 

 than that of endogens. Thus, e. g., Mr. Gruppy found about 

 twenty-eight seeds and fragments of plants capable of growing, 

 of exogenous plants in the rivers Thames and Lea ; while there 

 were only about fourteen, or just half the number, of endogenous 

 species 



Points op Similarity between Endogens and Exogens. 



Boots, Stems, and Pedicels.— The fact that all the floral organs 

 of endogens are constructed on identically the same lines as 

 those of exogens — their ternary symmetry f being the chiet 

 difference — undoubtedly points to a common origin. It is, how- 

 ever, more especially the vegetative system to which I would 

 first draw the reader's attt ntion. 



Now points of similarity of structure in different plants are 



* " The Kiver Thames as an Agent in Plant Dispersal," Journ. Linn. Soc., 

 But. \ol. xxix. p. 333. 



t I have elsewhere shown how this probably arises out of the J-phyllotaxis ; 

 and this, in turn, is a result of there being only one cotyledon. * Origin of 

 Floral Structures/ &c. p. 28. 



