29(5 JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



the earth and air. Though they have done this, they have more or less 

 retained the morphological characters which they acquired through the 

 direct action of water. 



Evidences from the Distribution of Monocotyledons. — That Mono- 

 cotyledons arose in the tropical or sub-tropical regions, where great 

 heat and moisture prevail, is inferred from the fact that by far the 

 greater number of orders or families of this class are now tropical or 

 sub-tropical. Thus Sir J. D. Hookee has shown that the average pro- 

 portion of the families of Monocotyledons and of Dicotyledons is 1 : 3'7 

 in tropical floras and 1 : 4*8 in sub-tropical ; the discrepancy increases 

 in colder zones so that only six families of Monocotyledons are repre- 

 sented in Arctic and Antarctic regions. 



The Degeneracy of Monocotyledons. — The first comparison to 

 be made between terrestrial Dicotyledons and all Monocotyledons is the 

 universal degeneracy of the latter. Many botanists have noticed this ; 

 for it is observable in all parts of the organism when contrasted with 

 the corresponding ones of land plants of the other class. This simpler 

 construction at first led to the belief that they must be a more 

 primitive group ; but a more critical examination proves that this 

 general simplicity is really the result of degeneracy through the direct 

 action of water, which causes all aquatic flowering plants to be alike in 

 this respect. This degeneracy had previously been somewhat over- 

 looked, but it at once destroys the theory of primitiveness of Monoco- 

 tyledons, for the more perfect and elaborate structure of Dicotyledons 

 could never have arisen out of the degraded and permanently fixed con- 

 ditions of aquatic plants. Morphological characters when long lost, 

 cannot be recovered, as will be proved later on. 



In my last lecture on the effects of water on plants (pp. 88-94) I 

 drew attention to the deteriorating influence of water upon roots, stems, 

 leaves, and reproductive organs. I propose now to show that a similar 

 degeneracy occurs in all Monocotyledons, whether they be aquatic or 

 terrestrial plants. 



The Effect of Water upon Roots. — The immediate result of allowing 

 seeds of any dicotyledonous terrestrial plant to germinate over water 

 is to arrest the radicles or primary roots as soon as they enter it. The 

 fact that all Monocotyledons are deprived of a tap-root, or if it con- 

 tinued for a short time, as in the maize or the date, it soon perishes, 

 was noticed so long ago as 1808,* for M. Poiteau quotes M. Eichard 

 as saying that " the almost sudden destruction of the radicle appears 

 to be a character of Monocotyledons." To this M. Poiteau adds: — 

 "If we pass in review all the monocotyledonous plants known, we 

 shall see that not one has a tap-root." 



It has also long been noticed that the root-cap has its formative 

 tissue independent of that forming all the rest of the tissues of the 

 root, I have shown how this feature can be readily induced in the 



* Mimoire sur VEmhryon des Graminees, dcs Cypcracees et du Nehnnbo 

 (1808). Par A. Poiteau. 



