Adaptation in the Tristichacece and Podostemacece. 541 



Siidbrasilien.' The conditions of life are so absolutely uniform that any 

 species can live in practically any place affected by these orders, though at 

 any one locality they will more or less group themselves each in the kind of 

 place that best suits it. 



It therefore appears to me in the highest degree improbable that either 

 primary root or secondary shoot thallus can be looked upon as an adaptation 

 to violent water, especially when we consider that one form is mainly 

 Asiatic, the other mainly American, while, further, they both share their 

 habitats with the very slightly modified Tristichas, and with the also slightly 

 modified Podostemons. As I have before stated, the adaptation, if adapta- 

 tion there be, is to shallow, not rushing, water, but even upon that it is as 

 well not to lay too much stress, because T. hypnoides and some species of 

 Podostemon, neither of them with any thalloid flattening, are as well suited 

 to, and grow in, the same shallow water as any of the thalloid forms. 



The only thing that it is safe to say, after 40 years during which opinion 

 has at first gone headlong in favour of adaptation and afterwards against it, 

 is that the small forms can live in any depth of water in which the condi- 

 tions of illumination will allow their life to go on, while the very large 

 forms are only to be found in the larger rivers. 



If all the extraordinary morphological differences between these plants 

 were to be regarded as adaptational to the extremely small or non-existent 

 differences in their conditions of life, the adaptation in land families living 

 under more variable conditions would have to be something positively 

 astounding. Or again if these differences are adaptational, why do we get 

 one kind in one country, another in another, though there are similar species 

 of the same genus living in both ? Or why in one genus (Hydrobryum) 

 do we get " root " thallus, in another (Lawia), living in the same place in the 

 same river, " shoot " thallus ? 



To pass on now to the flower. In the Tristichacese, which are the 

 less modified forms, it is simple and usually regular, some species of 

 Tristicha itself having only one or two stamens on the lower side. In 

 the Podostemaceai it is sometimes regular, but more often dorsiventral, 

 frequently to a very high degree. Now dorsiventrality has often been 

 supposed to be an adaptation to insect visits ; it is usually supposed only to 

 be found in lateral flowers and in flowers which stand horizontally when 

 open. But in the Podostemaceae it is at its highest degree, and that the 

 most extreme known in the higher plants, in flowers which are anemophilous, 

 terminal, and erect. In my monograph of the Indian forms I have gone 

 fully into this question, and may refer to that work for details. The con- 

 clusion there reached was that " the dorsiventrality of the flowers, which is 



VOL. LXXXVII. — B. 2 T 



