i8 93 . MONOCOTYLEDONOUS PLANTS. 133 



occasionally, as in Sisyrinchium, and sometimes in Iris, the apex of the 

 cotyledon, having functioned as a sucker, follows the lower part out of the 

 seed, the whole appearing above ground as a subulate green leaf in the 

 characteristic liliaceous fashion. In the Hydrocharideae and Naiadeae, 

 the hypocotyl and cotyledon are sometimes coherent into a more or 

 less oval mass terminated by a small radicle, but in others, as Stratiotes 

 and Halophila, the single cotyledon is distinct from the hypocotyl, and 

 in the former genus large and fleshy. It is also large and distinct in 

 Alisma. The absence of distinction between hypocotyl and cotyledon 

 crops up now and again in dicotyledons, and is not necessarily 

 connected with an aquatic habit. It has been already mentioned in 

 Utricularia, but is also well shown in the Brazil nut {Bevtholletia) and 

 another closely-related myrtaceous plant, Lecythis Zabucajo. 1 



The sheathing petiole so characteristic of the leaves of monocoty- 

 ledons, is next brought forward in support of the theory, as it also 

 obtains in many aquatic exogens, e.g., Ranunculaceae, Umbelliferae, 

 etc. It is especially characteristic of the Umbelliferae, but unless 

 we assume this an aquatic order, it seems equally well adapted 

 to land plants, especially herbaceous ones with large leaves, where it 

 will materially strengthen their attachment to the main axis. In the 

 enormous leaves of palms this factor must be a very important one ; 

 in fact, it seems to supply a more rational explanation than the 

 persistence of an adaptation to an aquatic habit. If such a habit is 

 the cause of the sheathing form of leaf, why is the form so extra- 

 ordinarily well-developed in the grasses, which is not an aquatic order. 

 The argument is, however, pushed still further, and it is suggested 

 that the coleorhiza, the sheathing portion which encloses the root in 

 grasses, is a development of the sheath, representing the decurrent 

 sheaths of two united cotyledons, or, perhaps, of one only completely 

 investing the axis. 



The " pseudo-coleorhiza " of Tropceolum is introduced as throwing 

 light on the origin of the root-sheath in endogens ; and the author 

 proceeds to adduce several points in favour of an ancestral aquatic 

 habit for the genus, as nowadays it is, unfortunately, a land plant. 



In the first place, however, as Mr. Henslow himself mentions, the 

 structure in Tvopczolum is formed from the bases of the cotyledons, 

 which are sagittate, being produced downwards on either side as four 

 lobes surrounding the embryo, so that if we suppose them to be 

 completely welded together, " they would form a perfect coleorhiza 

 over the end of the radicle " ; but this is no argument in favour of the 

 origin of the coleorhiza from a sheathing petiole. 



Thus we cannot agree with the statement with which the first 

 part of the paper concludes, viz., " that there is at least enough pre- 

 sumptive evidence to frame a theory that the monocotyledonous 

 embryo has been derived from a dicotyledonous one by a suppression 

 of one cotyledon" ; the cause of the suppression being " the degene- 

 1 Cf. Lubbock's " Seedlings," vol i.,pp. 539-41- 



