MIMETIC RESEMBLANCES AMONG PLANTS. 



329 



take with reference to incident lijo^ht. Indeed, experiments have shown 

 that leaves will be broader or narrower as the amount of light is greater 

 or less which is allowed to fall upon them. 



Aquatic Types. — The form most common in Dicotyledons which have 

 submerged leaves is for them to be 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 the Water Crowfoot (Ranunculus 

 heterophyllus). 



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

 as in Cabomba (Nymphaeaceae), Myrio2)hyllum (Halorageas), Hottonia 

 (PrimulacesB), Apium inundatum (Umbelliferge), Cemtophyllum (Cerato- 

 phylleaB), &c. Affinity among these plants is quite out of the question, 

 but since the Water Crowfoot is undoubtedly descended from some terres- 

 trial Buttercup, and the Haloragece are aquatic forms of Oimgracece, and 

 Hottonia is allied to land Primroses, &c., the inductive evidence is ample 

 to prove that tliis type of submerged foliage is entirely due to the direct 

 and arresting action of the aquatic medium, which brings about degrada- 

 tions, not only in the leaves, but throughout the entire plant. 



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

 Lobelia Dortmamia, Hippuris, &c., 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 long strap-shaped phyllode develops a blade at the 

 summit. This is at first oval, then it becomes hastate, and finally sagittate 

 when completely out of water, giving the name " Arrowhead " to the 

 plant. 



Now it has been found that when a phyllode was on the point of 

 developing the oval blade at the summit and the water became suddenly 

 deepened, it at once began to grow out further into the linear form. 

 Indeed, the writer has a specimen in which the three points of the 

 sagittate leaf have elongated into long narrow ribbon-like extremities, 

 doubtless from the same cause. 



Turning to the NymphceacecB of Dicotyledons, the development of the 

 leaves of Victoria regia follows a similar procedure. The first leaves only 

 develop phyllodes, then a hastate blade is produced, and finally an 

 orbicular one which floats on the surface of the water. 



In the Lotus {Nelumbium) the lower lobes unite and a peltate leaf is 

 produced. In Monocotyledons the rounded stage in the development of 

 the leaf is seen in the Frog-bit, and the united basal portions of the 

 hastate leaf in Caladium &c. resemble a similar union in the Lotus. 



Taking all these and other facts into consideration, the conclusion is 

 inevitable that similar structures, often mimetic in form, seen in aquatic 

 flowering plants are simply the result of self-adaptation to the aquatic 

 medium in which the plants live. 



Specialised Mimetic Organs. — In the preceding cases the connection 

 between the plant-structure and the influence of the environment is 

 obvious ; but in those which follow, though one may be convinced that 

 like causes have produced like eft'ects, it is not so easy to say why any 

 particular structure occurs in preference to another, which might have 

 been equally serviceable. 



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