i2 4 GEORGE HENSLOW [fbbkuaet 



twigs more exactly. If so, there is both an unconsciously acquired 

 resemblance in form and a conscious mimicry in action. 



If, therefore, mimicry has been thus allowed to have its meaning- 

 extended to include a superficial or structural resemblance in animals, 

 often apart from imitation in habit, we are quite justified in applying 

 it to plants, among which such resemblances abound. 



In the vegetable kingdom it arises from common causes and issues 

 in common uses. 



Mimetic resemblances are particularly characteristic of plants 

 growing in very specialised environments, such as under water ; in 

 high alpine, arctic, or antarctic regions ; in deserts and arid countries, 

 etc. ; as well as of plants which are highly specialised for carrying on 

 certain peculiar functions, as of insectivorous, climbing, parasitic 

 plants, etc. 



In all these the same functions have resulted respectively in pro- 

 ducing similar structures. In other words, whether one studies the 

 external mimetic morphology or internal anatomy, the conclusion 

 appears to be inevitable that these imitative features are simply due 

 to the direct action of the environment, together with the responsive 

 powers of protoplasm ; though one cannot explain how the process is 

 effected. 



Aquatic Types. — Commencing with submerged leaves, the type 

 of foliage most common in dicotyledons is finely dissected, though 

 as soon as the stem reaches the surface of the water it may develop 

 more or less lobed or entire leaves, as may be seen in Ranunculus 

 heterophyllus and the water-lily. 



Now this type is imitated by many aquatic plants of no affinity, 

 as in Cabomba (Nymphaeaceae), Myriophyllum (Halorageae), Hottonia 

 (Primulaceae), Apium inundatum (Umbelliferae), Ceratophyllum (Cera- 

 tophylleae), etc. Affinity among these plants is quite out of the 

 question ; but since the water crowfoot is undoubtedly descended 

 from some terrestrial buttercup, and the Halorageae are aquatic forms of 

 Onagraceae, and Hottonia is allied to land primroses, etc., the inductive 

 evidence is ample to prove that this type of submerged foliage is 

 entirely due to the direct and arresting action of the aquatic medium, 

 which brings about degradations throughout the entire plant. 



Another type of submerged leaf is ribbon-like. This is seen in 

 Lobelia dortmanni, Hippuris, etc., but it is commoner among mono- 

 cotyledons. From these, as in Sagittaria, we learn that this form is 

 really phyllodinous as long as it is in deep water ; but when the 

 surface is reached the leaf develops a blade at the summit. And since a 

 progress from phyllodes to sagittate blades is imitated in Nymphaeaceae, 

 we a^ain see a direct cause and effect. 1 



Juncaceous Type. — A stem with much reduced leaves and a rounded 



1 For fuller details of these and the following types the reader is referred to "The 

 Origin of Plant Structures," Internat. Sci. Series, vol. lxxvii. 



