[] THE ORIGIN OF THE GROUP 133 



he marine Potamogetonaceae, the epidermis is characteristi- 

 :ally free from stomates and very rich in chlorophyll. Liquid 

 ;xchange between the plant and the surrounding medium is 

 "acilitated by the occurrence of openings at the leaf apices, with 

 vhich the median nerve is in direct communication. These 

 )penings come into existence quite early in the history of the 

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

 ^enus Posidonia^ again, the fibres of the sheath survive and 

 "orm a protective covering for the younger leaves. Another small 

 Deculiarity, which may be adaptive or may more probably be 

 in indication of community of origin since it is common to 

 :ertain genera in the two families under consideration, but is 

 lot found elsewhere is the occurrence of " Flossenzahne " 

 )r "dents nageoires " on the leaf margins^. These teeth are 

 ormed by a peculiar elongation and wall-thickening of the 

 narginal cells. 



The fact that a considerable number of Phanerogams live 

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

 narine types all prove to be restricted to representatives of two 

 elated families, stimulates conjecture as to the origin of this 

 biological group. Both the families to which the flowering 

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

 epresented in fresh waters ; no marine Angiosperm has a close 

 iflinity with any terrestrial plant. These facts suggest that the 

 lowering plants now living in the sea are not the immediate 

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

 :ors which had already accommodated themselves to life in 

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

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

 "acuities. These are, firstly, toleration towards a saline medium; 

 secondly, the power of vegetating while wholly submerged; 

 ;hirdly, the knack of developing a sufficiency of anchoring roots 

 :o withstand the wash of waves and tide; and, fourthly, the 

 :apacity for hydrophilous pollination, since any aerial method 

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

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



