May, 1905.] Ohio Plants with Compound Leaves, 341 



2. By increase in height or length, thus providing room for a 

 greater number of leaves, as in trees, vines and ivies. 



3. By leaf arrangement in which the plant secures the best 

 possible light relation by arranging the material on hand to its 

 best advantage. Here we have rosettes, mosaics, etc. 



4. By an increase in the size of the leaf blade or the length- 

 ening of its petiole. When the petiole is lengthened it is only 

 to place the leaf in a better position, so this phase of leaf enlarge- 

 ment would more properly come under No. 3. When an enlarge- 

 ment of the blade takes place it must be in such a manner that 

 nothing will be sacrificed to light or strength and it is evidently 

 for this reason that we get the great variety of forms which 

 gradually lead up to the compound leaf. 



In our common Monocotyls leaf enlargement takes the form 

 of increase in length only ; this being necessary on account of the 

 parallel system of venation which could not prevent the leaves 

 from becoming shredded when exposed to wind and rain, some- 

 thing which does take place in a great many palms. Some of 

 the palms which have pinnately compound leaves and the aroids 

 present quite an exception to this statement however, as their 

 leaves are usuall}' quite large and expanded. Our common 

 Arisaemas are very distinctly palmately compound. 



Among Ferns and Dicotyls we have the greatest variety of 

 forms ranging from those that are but slightly toothed or lobed 

 to those which are deeply lobed, cleft or divided, until finally the 

 division is so marked that we have a compound leaf apparently 

 made up of separate leaves on a common axis and petiole. That 

 these compound leaves are a gradual development from simple 

 ones may be readily observed by comparing the leaves of differ- 

 ent species in the same families or genera; those of different 

 individuals in the same species and finally the older and later 

 leaves on a single individual. Leaves that are pinnately veined 

 will give rise to pinnately compound ones, while those which are 

 palmately veined will give rise to leaves that are palmately com- 

 pound. Of the ferns, Botrychium affords the best example of 

 compound leaf development from the simpler forms like B. 

 lunaria to the highly complex leaf of B. virginianum. In differ- 

 ent species of Ranunculus all transition forms are also easily 

 observed. Often another feature is added here. In some forms 

 the plants have the rosette habit while young and when leaves 

 are few ; later the leaves become compound and thus avoid shad- 

 ing the older ones. The writer has before him a seedling of 

 Robinia, in which the first true leaf is a simple one; the second 

 and third are each composed of three leaflets; the fourth, fifth 

 and sixth each of five ; and the seventh and eighth each of seven 

 leaflets. 



