EXTERXAL COXDITIOXS OF PLAXT LIFE. 1 8 



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has adapted itself to a growth between tide waters. In so 

 doing it not only produces the remarkable, vertical, negatively 

 geotropic roots, which grow up out of the salt mud, so that 

 at low tide they remain each day several hours in the air for 

 oxygen absorption, without which it could not live, but it has 

 also remarkably thickened its leaves and appressed them 

 vertically against the stems in order to protect itself from loss 

 of water, for reasons which have been already stated. 



From the history and general relation of this mangroove 

 to the Verbena family, we judge that this adaptation is of 

 comparatively recent date. When removed to dry cultivation 

 it loses all these characteristics in the first offspring. In the 

 case of the Cypress we have not merely theoretical reasoning 

 from family relationship, but also definite data from fossil 

 remains, which prove that it was a dry land form in a very 

 recent geologic age, and that its aquatic habits are modern. 

 It is certainly curious, and you may interpret it as you please, 

 that both these plants, when subjected to opposite conditions, 

 immediately lose their acquired characters. In the Australian 

 plants, on the contrary, similarly acquired characteristics under 

 opposite conditions are perfectly permanent. That they have 

 been in this condition for an immensely longer period of time, 

 and that in consequence this development is so stamped upon 

 their parent stock that it reappears generation after generation, 

 even when withdrawn from their causal surroundings, is, 

 perhaps, the key to this riddle. 



Let me, then, remind you once more of the almost infinite 

 variety and diversity of plant forms, and that, as seen in 

 nature, we can generally trace the relationship between the 

 peculiarities of the plant and its surroundings ; also, that in 

 general, these peculiarities, no matter what they may be, 

 reproduce themselves in the offspring. Let me state once 

 more that any plant which we may select as an example, no 

 matter how peculiar in form or habit of growth, gives us an 

 expression in this peculiar form and special habit of the forces 

 which have surrounded it and touched it all the way down its 

 long line of descent, from its earliest more simple and primary 

 condition to its present more complete expression of plant life. 



