x] THE ORIGIN OF THE GROUP 133 



the marine Potamogetonaceae, the epidermis is characteristi- 

 cally free from stomates and very rich in chlorophyll. Liquid 

 exchange between the plant and the surrounding medium is 

 facilitated by the occurrence of openings at the leaf apices, with 

 which the median nerve is in direct communication. These 

 openings come into existence quite early in the history of the 

 leaf, and are due to the disappearance of the epidermis. In the 

 genus Posidonia, again, the fibres of the sheath survive and 

 form a protective covering for the younger leaves. Another small 

 peculiarity, which may be adaptive or may more probably be 

 an indication of community of origin since it is common to 

 certain genera in the two families under consideration, but is 

 not found elsewhere is the occurrence of " Flossenzahne " 

 or " dents nageoires " on the leaf margins 1 . These teeth are 

 formed by a peculiar elongation and wall-thickening of the 

 marginal cells. 



The fact that a considerable number of Phanerogams live 

 and flourish in the sea, and that yet, on examination, these 

 marine types all prove to be restricted to representatives of two 

 related families, stimulates conjecture as to the origin of this 

 biological group. Both the families to which the flowering 

 plants of the sea belong are typically aquatic, and are widely 

 represented in fresh waters ; no marine Angiosperm has a close 

 affinity with any terrestrial plant. These facts suggest that the 

 flowering plants now living in the sea are not the immediate 

 descendants of land plants, but have been derived from ances- 

 tors which had already accommodated themselves to life in 

 inland waters. It would seem that, in order to be capable of em- 

 barking upon life in the sea, a flowering plant requires four special 

 faculties. These are, firstly, toleration towards a saline medium; 

 secondly, the power of vegetating while wholly submerged; 

 thirdly, the knack of developing a sufficiency of anchoring roots 

 to withstand the wash of waves and tide; and, fourthly, the 

 capacity for hydrophilous pollination, since any aerial method 

 must be doomed to failure, except in halcyon weather in a non- 

 1 Ascherson, P. and Graebner, P. (1907). 



