248 SEEDS OF AQUATICS [ch. 



In the case of Farmer i a metzgerioides, one of the Podostemaceae, 

 germination of the two-seeded, indehiscent fruit also occurs in 

 situ^. 



The seedUngs belonging to certain floating plants owe their 

 station at the water surface to the early development of some 

 type of buoyant organ : in the case of Lemna^ for instance, the 

 cotyledon itself acts as a float (Fig. 52, p. 81). The seedlings 

 of certain plants which are rooted at maturity, are capable of 

 developing to a considerable extent 

 while still unattached. Some seeds of 

 Limnanthemum nymphoides'^ were kept 

 in water over a winter by the present 

 writer, and on February 11, one of 

 them was observed to have germinated 

 while floating. 



It is a somewhat remarkable fact 

 that the large group of the Monocoty- 

 ledons which are known collectively as 

 the Helobieae or Fluviales the Alis- 

 maceae, Butomaceae, Hydrocharita- 

 ceae, Juncaginaceae, Aponogetonaceae, 

 Potamogetonaceae and Naiadaceae 

 are uniformly characterised by the 

 absence of endosperm and by a 'ma- 

 cropodous' embryo, in which the hypo- 

 cotyl reaches excessive proportions (e.g. 

 Zannichellia^ Fig. 159, p. 246, Zostera^ 

 Fig. 161, Ruppia^ Fig. 1 66, p. 3 1 9) ; in 

 almost all other features the members 

 of the group show great range and 

 diversity. Except the Helobieae, the Monocotyledons may be 

 said, in general, to be characterised by the possession of endo- 

 sperm. The surmise suggests itself that possibly there may be 

 some connexion between water life and an exalbuminous seed 

 with an enlarged hypocotyl. The predominance among aquatics 



1 Willis, J. C. (1902). 2 Fauth, A. (1903). 



Fig. 161. Znstera marina, L. 

 Fruit in longitudinal section. 

 (X15.) /^=fruit coat; /s= 

 seed coat; r= primary root; 

 y/!= hypocotyl which at its 

 base enwraps the cotyledon a. 

 [Raunkiaer, C. (1896).] 



