S6o 



ECOLOGY 



The arrangement of stomata. — As a rule, stomata overlie the mesophyll 

 rather than the veins, and commonly they number from loo to 300 per 

 square millimeter. In most mesophytic herbs, stomata are found on 

 both leaf surfaces, but rather more occur below than above. In most 



monocotyl leaves the stomata are in longi- 

 tudinal rows and have a common orienta- 

 tion, their long axes coinciding with that of 

 the leaf (fig. 804). In most dicotyl leaves 

 the somewhat more numerous stomata are 

 scattered irregularly and their long axes 

 are oriented in various directions (fig. 911). 

 In some broad monocotyl leaves the stomata 

 are arranged and oriented irregularly, while 

 in many narrow dicotyl leaves they occur 

 in rows, thus suggesting that leaf shape 

 and venation rather than systematic position 

 may be the chief determining factors ; in 

 Salsola and in other halophytes the stomata 

 are in rows and are transversely oriented; 

 in Saxifraga granulata the broad basal 

 leaves exhibit irregular orientation, while 

 the narrow upper leaves have the regular 

 orientation characteristic of monocotyls. 



The leaves of most trees, whether meso- 

 phytic or xerophytic, are without stomata 

 on the upper surface; in Juniperus the 

 stomata are confined to the upper surface, 

 but that is the least exposed surface during 

 the winter (p. 582); in Populus deltoides the 

 stomata are about equally abundant on the 

 two surfaces, but the constant trembling of 

 the leaf frequently exposes the under side 

 to the sun. In grasses the stomata usually 

 are confined to the upper surfacs, which is 

 ihe more protected side in dry weather, owing to the infolding of the 

 leaves (figs. 835-837). Except in a few instances (p. 564) submersed 

 hydrophytes are without stomata, so that their large air chambers 

 have no direct outside connection. In floating foliage organs, such as 

 water lily leaves (fig. 805) and duckweeds, the submersed under 



Fig. 804. — A surface view 

 of the leaf epidermis of Anthe- 

 ricum, showing the uniform 

 orientation and the regular ar- 

 rangement of the stomata that 

 characterize most monocotyls, 

 the long axes of the epidermal 

 cells being parallel to the long 

 axis of the leaf; the straight epi- 

 dermal walls contrast with the 

 wavy walls of many dicotyls; 

 considerably magnified. 



