F. W. WENT 289 



surface development of mesophyll. By following these tendencies 

 throughout the angiosperms we find that there are two major leaf types 

 which behave differently upon changes in proportions of caulocaline 

 and phyllocaline. These are the palmate and the pinnate leaves. To the 

 latter type belongs also the parallel-veined leaf. In a pinnate leaf a 

 decrease in the amount of phyllocaline results in a narrowing of the 

 distance between the veins, which thus make a smaller angle with the 

 midrib. This results in a narrower leaf. Yet the veins can retain the same 

 length if the same amount of caulocaline is available. In a palmate leaf, 

 on the other hand, the major palmate veins retain the same angle, but 

 a decrease in phyllocaline results in lobing. 



Morphology. — There are a few outstanding examples of differential 

 development of leaves which are mentioned in all morphology text 

 books. As a first case we will discuss the form differences between sub- 

 merged and floating leaves in certain water plants. If we start with the 

 monocotyledons we see there that leaves of Sagittaria, Alisma Plantago, 

 A. nutans are all linear if they grow under water, which is the case in the 

 young leaves and in older plants which are grown in streaming water. 

 Under those conditions these species can hardly be recognized one from 

 another, but the first leaves which reach the surface of the water and 

 either float on it or stand up above it develop a pronounced blade by 

 widening of the apical part of the leaf. In successive leaves this widening 

 increases until the typical form is reached, but the plant reverts to its 

 linear type of leaves as soon as it is submerged again. In Potamogeion 

 natans and P. heterophyllus the same difference between narrow sub- 

 merged leaves and wide floating leaves occurs. In dicotyledonous plants 

 with palmate leaves the submerged leaves are not linear themselves, 

 but the leaf is divided into linear segments such as in Cabomba, Lim- 

 nophila, and Batrachium. In those leaves mesophyll development is 

 practically lacking. The floating leaves, however, are entire and have 

 good mesophyll development so that the whole space between the veins 

 is filled. In both of these cases linear leaves or linear lobes must be 

 attributed to a lack of mesophyll growth factors, whereas the vein 

 growth factors are normally present. Another tempting conclusion would 

 be that in these plants the mesophyll growth factors diffuse out into the 

 water just as they diffuse out of germinating peas if these are submerged 

 (5.38).* 



*How general this phenomenon is of leaf reduction in submerged leaves, is in- 



