CH. II] THE PODOSTEMACEAE 19 



caught in some fragment of old plant, or in some other place 

 where it can germinate. 



At the period when this study was undertaken, the Podo- 

 stemaceae, with their strange look of lichens or seaweeds, their 

 peculiar mode of growth, their great variety of form, were looked 

 upon as obviously showing adaptation in the highest degree, and 

 it was for this reason that the work was undertaken. But among 

 the conclusions drawn from it was this, that apart from those 

 adaptations which they showed in common with all water plants, 

 such as the lack of strengthening tissues and of stomata, there 

 was in them little evidence of any special adaptation whatever. 

 The conditions under which they lived were the most uniform 

 that it was possible to conceive — the same mode of life, no com- 

 petition with other forms of life, the same substratum, the same 

 light (varying from day to day with the depth of the water), the 

 same temperatures, the same food, everything the same. Yet in 

 spite of this, the plants showed an enormous variety of form, 

 greater than that of any other family of flowering plants what- 

 soever, while water plants as a rule show little variety in form, 

 and have but few genera and species. Still more remarkable was 

 it that their morphology diff'ered for each continent, flattened 

 roots in the Old World, flattened shoots in the New, so that it was 

 usually possible to say by a simple inspection what was the 

 probable habitat of a species never seen before. It was hard to 

 believe that natural selection, working upon structural modifica- 

 tions that have never been shown to have any functional value, 

 could do this. The linking genus, Podostemon itself, covers an 

 immense area, including that of many of the smaller genera, and 

 is less dorsiventral than they are, though all show a highly dorsi- 

 ventral flower, which stands erect, and is commonly wind 

 pollinated, an unexpected combination of characters for the 

 selectionist to explain. 



Once these very remarkable facts were fully realised, the 

 explanation that seemed much the most probable was that on the 

 whole the highly dorsiventral genera were descended from Podo- 

 stemon or from some form like it. It could not be the other way 

 about, for the flowering plants as a rule are not dorsiventral, 

 except in the structure of the leaf, and very often in the flower. 

 Nor could there have been some intermediate form, for that 

 would have had to be more dorsiventral than are the flowering 

 plants in general. One only of the local genera, Willisia in the 

 Anamalai mountains in South India, shows as much ordinary 



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