14 



SCIENCE. 



[N. S. Vol. VII. No. 158; 



cells, and communicate directly with their 

 intercellular spaces. 



Every stoma is more than a mere slit be- 

 tween the epidermal cells. It is, in fact, a 

 simple organ consisting of two active cells, 

 the ' guard cells,' between which is the elon- 

 gated opening. By changes in shape the 

 guard cells narrow or broaden the opening, 

 or completely close it. 



It has been found that gases and water- 

 vapor pass through the open stomata. In 

 the case of gases the passage is in either di- 

 rection, while it appears that the water- 

 vapor passes in one direction only, namely 

 from the intercellular spaces outward. From 

 the fact that the stomata serve for the pas- 

 sage of both gases and water-vapor have 

 arisen two views as to their proper function, 

 some botanists holding that they are organs 

 of respiration, that is, breathing pores, while 

 others regard them as transpiration organs, 

 that is, organs for permitting the escape of 

 surplus water from the tissues of the plant. 

 According to the first view the stomata are 

 connected directly with the process of photo- 

 syntax and the metabolic changes which 

 follow it, in other words, with the supply 

 of carbon to the plants, while according to 

 the second view they are connected with 

 the supply of inorganic salts to the ash con- 

 stituents of .the plant. Since the passage 

 of water-vapor through the stomata is a 

 much more noticeable phenomenon than the 

 ingress or egress of gases, it is quite nat- 

 ural that at first the former should be con- 

 sidered as the primary function. With this 

 view have come corresponding explanations 

 of the purpose of transpiration, involving 

 much of the discussion of nutrition in the 

 treatises on plant physiology. If stomata 

 are organs of transpiration, then transpira- 

 tion is a physiological phenomenon of much 

 importance, and it behooves us to find out 

 why plants have developed organs for its 

 promotion. 



In considering the questions involved, it 



is well to remember that terrestrial plants 

 which possess stomata have developed from 

 aquatics none of which h ave stomata . These- 

 aquatics, living in the ocean or the fresh- 

 water rivers and lakes, must supply them- 

 selves with all their food constituents from: 

 the water and the substances it holds in 

 solution. In the simpler plants every cell 

 absorbs these directly from the surrounding 

 water, and this is true of the larger plants 

 also, with slight modification. We must 

 not, however, overlook the fact that water 

 itself is an indispensable constituent of every 

 cell, not as food, but as a part of its me- 

 chanical structure. More than nine-tenths 

 of every active cell is water, upon whose 

 presence the activity of the cell is depend- 

 ent. In aquatics this necessary water is 

 supplied directly from the surrounding 

 medium, and since there is no loss of water 

 each cell easily maintains all that it re- 

 quires. 



Terrestrial plants must supply their cells 

 with the necessary food constituents, and 

 must, also, maintain in them the proper 

 amount of water. Every cell in a terrestrial 

 plant must be turgid with water in order 

 to be active, and if this be impaired the 

 plant suffers. The maintenance of the- 

 water supply is thus of the greatest im- 

 portance in terrestrial plants. Accord- 

 ingly, the roots are always in communica- 

 tion with water in the soil from which they 

 obtain their supply. The cells of the stems 

 and leaves must obtain their water by ab- 

 sorbing it from the turgid root-cells. Now, 

 these cells in the stems and leaves not only 

 have no direct access to water, having to- 

 obtain their supply indirectly, at second 

 hand as it were, but they are surrounded 

 by a medium which is drier than they, so- 

 that they are constantly losing water by 

 evaporation. This loss of water is usually 

 greater than the scanty supply from the 

 water of the soil, and accordingly the aerial 

 parts of plants are protected by a layer of 



