No. 62G] SHORTER ARTICLES AND DISCUSSIOX 



273 



b t lof 11( 

 of the St .Ikts 



111(1 p-owing above about the toppe 

 )ut the small jaj^ged leaves. That 



oveiwlicliiicd witli wat.'r. Imtli also at the top of the stalkes. small 



Among certain NymplneacesB we find a different type of hetero- 

 phylly in which the submerged leaves are large, thin and translu- 

 cent, somewhat resembling the seaweed Ulva. These leaves are 

 particularly well shown in the yellow water-lily. 



To enumerate all the varieties of submerged leaf met with 

 among angiosiperms w^ould be too long a task to undertake in the 

 present paper. It must suffice to say that they are either highly 

 divided, ribbon-like, or else thinner and broader than the cor- 

 responding air leaves. They are characterized anatomically by 

 the lack of stomates and by the presence of chlorophyll in the 

 epidermis. They are thus well suited for the aTasorption of car- 

 bon dioxide in the dissolved form in which it presents itself to 

 water plants. 



In considering the significance of heterophylly, it is a matter of 

 importance to remember that the occurrence of different leaf- 

 forms in a single individual is not confined to aquatics but occurs 

 also in terrestrial plants, Nehemiah Grew, as long ago as 1682, 

 pointed out that in many eases one plant bears leaves 

 of Two Kinds or Two distinct Figures; as the Bitter-siveet, the common 

 Little Bell, Valerian, Lady-Smocks, and others. For the Under leaves 

 of Bitter-Sweet, are Entire; the Upper, with two Lohes : the Under 

 Leaves of the Little Bell, like those of Fancy; the Upper, like those of 



"We find parallels to the heterophylly of hydrophytes, not only 

 among terrestrial flowering plants, but also in the case of the dis- 

 tinct "youth forms" of conifers, and even — more remotely — in 

 the " Chantransia" stage of such alga? as BatracJiospermum. 

 Heterophylly is indeed so widespread that no interpretation can 

 be valid unless the condition be treated broadly as a very general 

 attribute of plant life, rather than as a rare and exceptional phe- 

 nomenon, for which special and individual explanations will 

 suffice. 



To the earlier writers, such as Laniart-k, tlie problem of hetero- 

 phylly presented no difficulties. They regarded the su])merged 

 or aerial type of leaf as representing a direct response, on the 



