ORIGIN OF EXDOGEXS FROM EXOOENS. 523 



Thus, just as Ranunculus heterophyllus readapts its dissected 

 leaves for an aerial existence without refilling up the interstices 

 with parenchyma, as in the floating leaves, so analogous condi- 

 tions may occur in endogens. In submerged leaves of endogens, 

 since the ribs run from end to end, the dissected type is not pos- 

 sible ; but fenestration represents it. As this has now become 

 an hereditary character in Ouvirandra, it is not surprising to find 

 a similar feature retained in some terrestrial endogenous plants 

 at the present day, as in the Aroids Tornelia, Monstera, &c, 

 which are presumably descended from aquatic ancestors ; as this 

 feature is not due to " tearing " as in banana-leaves, but to an 

 arrest of development of the cellular parenchyma. 



If such plants are grown in a luxurious soil, the number of 

 cavities decrease or cease altogether, showing that thev are due 

 to a deficiency of nutriment, but the " habit " of producing them 

 has become hereditary*. 



Air-cakals. — These occur in all w r ater-plants, are of symme- 

 trical construction, often with peculiar diaphragms, and are 

 found in exogens and endogens alike. Consequently it is justi- 

 fiable to assume them to be adaptational to an aquatic existence. 

 When, however, water-plants become terrestrial, they may be 

 retained, as is the case with Ranunculus Ficaria, Caltha palus- 

 tris, and JZhizophora. Similarly they are found in Pandanus, 

 Strelitzia, Musa, &c, which no longer grow with their petioles 

 and stems submerged ; consequently, the inference is that those 

 plants have descended from truly aquatic ancestors. 



Water-glands. — There is yet another feature which sepa- 

 rates Dicotyledons from Monocotyledons, which Dr. Yolkens 

 observed, namely, that in the former " there is usually present 

 a well-developed water-gland, possessing several layers of 

 1 epithem ' tissue interposed between the tracheides, into which 

 the spiral vessels pass, and the epidermis. In Monocotyledons 

 the tracheides frequently end directly beneath the epidermis, or 

 are at most separated from it by one or two layers of cells only, 



which never, so far as could be observed, show that marked dis- 



* It may be remarked that accidental checks to growth by frost often pro- 

 duce them in the leaves of expanding buds of trees ; so that when the leaves 

 become adult they appear perforated and subpinnate, when they would naturally 

 be entire. 



