1902.] Influence of an Excess of Carbon Dioxide on Plants. 415 



proportion existing between the epidermal cells and stomata as a 

 consequence of the added carbon dioxide. It will, however, be clear 

 that the small leaves of the latter series do possess, area for area, 

 a far greater number of stomata than do the leaves of the control 

 plants. 



a. b. 



Fig. 1. Kalanchoe WelwitscMi, epidermis of the underside of the leaf. A, air- ; 



B, COo-plant. 



In the anatomical or internal structure the differences are not so 

 great as might perhaps have been anticipated. They chiefly affect the 

 amount of xylem vessels and tracheids produced, and also the degree 

 of differentiation of the mechanical tissues. 



The vascular bundles are commonly less in number in the CO2 than 

 in the control plants, and the xylem of the former is always more 

 parenchymatous in character than is that of the latter series in which 

 the tracheids and vessels are tolerably uniformly distributed (see fig. 2). 

 The phloem is, however, equally well developed in the bundles of both 

 series. The stem of the air-grown plants is provided with a well- 

 marked hypodermal collenchyma and pericyclic sclerenchyma, and in 

 both respects it surpasses the CO2 plant in the degreee of development 

 marked by these tissues. We consider that the reduction in the 

 conducting elements of the xylem is probably to be chiefly correlated 

 with the diminished leaf surface, and the consequent lower transpira- 

 tion, for the total number of stomata on the small leaves is far short 

 of that present on the normal plants. The comparative poverty in 

 mechanical tissue is perhaps partly connected with nutritional dis- 

 turbances, and partly with the diminished weight of the growing 

 organs of the plant. For it is known that the degree of differentiation 

 of mechanical tissues is subject to modifications depending on the 

 amount of stress or strain imposed upon the growing organs. 



2 f 2 



