Januaey 7, 



B.] 



SCIENCE. 



15 



-cuticularized cells, the epidermis. The 

 ■drier the air in which a plant grows the 

 ■thicker the epidermal layer, an extreme 

 •case occurring in the Burro Thorn {Solo- 

 'CantJia emoryi) of the arid regions of south- 

 ern Arizona, where there are from three to 

 five layers of cells in the epidermis. That 

 plants are able to protect themselves against 

 very dry air is shown by the fact that even 

 in excessively dry climates there are many 

 •species which are able to live and form 

 ilowers and seeds. 



Bui with the change from the aquatic to 

 to the terrestrial habit there came a divi- 

 sion of labor in the organs of absorption. 

 The roots now absorb water and solutions; 

 while the stems and leaves absorb carbon 

 ■dioxide. And here arises a difficulty: The 

 epidermis which prevents the escape of wa- 

 ter-vapor also prevents the absorption of 

 <3arbon dioxide. This difficulty was sur- 

 smounted by the formation of stomata. A 

 leaf without stomata, or what is the same 

 thing, with its stomata permanently closed 

 as with wax, will not lose water, but it will 

 starve for want of carbon dioxide. These 

 stomata are open as long as there is no 

 danger of such a water loss as would result 

 in loss of turgidity, but when the cells 

 show an approach to flaccidity the stomata 

 •close. While open there may be a free in- 

 terchange of gases, carbon dioxide entering 

 ^nd being absorbed by the chlorophyll-bear- 

 ing cells, but while this is going on there 

 ds certain to be a considerable loss of water, 

 especially if the air be dry. On every dry 

 ■day land plants lose much water, since they 

 must have their stomata open in order to 

 •obtain their supply of carbon dioxide. 



Aerial plants, as many Tillandsias and 

 Orchids, do not differ in any essential re- 

 spect from terrestrial plants. They must 

 have enough water to keep their cells turgid 

 and, at the same time, their chlorophyll- 

 bearing cells must be supplied with carbon 

 -dioxide. They invariably grow in moist 



climates, where the constant moisture of 

 the air is supplemented by frequent drench- 

 ing rains. Under such conditions many 

 terrestrial plants would be able to live and 

 grow for some time. At the same time it 

 is to be observed that many aerial plants 

 have a greatly thickened epidermis, or have 

 their surfaces covered by a coat of dry hairs. 

 They evidently have taken some precautions 

 to guard against harmful water loss. 



It is not too much to say that the facts 

 cited above indicate that respiration is the 

 normal function of stomata, and that the 

 loss of water through stomata is incidental 

 and secondary. Some experimental results 

 may be cited here. 



a. Stahl has shown that when the sto- 

 mata are closed no starch is made,* show- 

 ing that the carbon dioxide must enter by 

 the stomata. 



b. Blackman concludes that " practically 

 the sole pathway for carbon dioxide into or 

 out of the leaf is by the stomata. "f 



c. Stahl has shown that transpiration 

 takes place through the stomata, and this 

 only when they are open. J 



d. Observations often repeated by many 

 physiologists show that the stomata of many 

 cultivated plants close quickly when the 

 supply of water to the roots is deficient, and 

 that plants in dry climates have remark- 

 able devices for preventing the loss of wa- 

 ter. 



e. Stahl has shown§ that in many ever- 

 green plants the stomata close during the 

 period when there is no carbon assimila- 

 tion. 



/. It is a well known fact that stomata 

 are usually open in sunlight, when carbon- 

 assimilation (photosyntax) is possible. 



g. Plants from which carbon assimila- 



*Bot. Zeit., 52: 127-133 (1894). 

 t Phil. Trans. Roy. Soo. London, 186, B: 485 (1895) ; 

 from abstract in Bot. Gaz., 20: 336. 

 t Bot. Zeit., 52: 117-127 (1894). 



n. c. 



