Structural Botany : Transpiration. V. 



The Gaseous Exchanges of the leaf may be expressed as : 



(1) CO 2 in, and O 2 out, in Photosynthetic Assimilation : the oxygen diffusing out 



passively when in excess. 



(2) O 2 in, and CO 2 out, in Respiration; CO 2 passively diffusing out in absence of 



anything to stop it. 



(3) H 2 O lost to the tissues in evaporation and Transpiration : all gases diffuse out 



if apertures are available, when in excess ; or may pass away from solution in 

 aqueous media : diffusion inwards continues as long as the gas is used chemi- 

 cally, and so creates a reduced internal partial pressure. 



Aeration of the Tissues is provided for by intercellular spaces, always of 

 secondary origin, in free communication throughout the plant as an internal atmos- 

 phere ; e. g. the gas-content of a potato is the same as that of the external air. The 

 paths of exchange are indicated by the stomata as intercellular spaces in the epidermis, 

 formed as a slit between twin guard-cells: cf. Theory of Multiperforate diaphragm. 



Stomatal Control : Penetration by gases, or outward diffusion, may be com- 

 pletely checked by an impermeable (gas-proof) surface-layer ; the normal function of 

 the cuticle, if only thick enough : but this does not give control. Stomata, with guard- 

 cells, opening and closing the slit, act as doors controlling the exchanges if the 

 mechanism is suitably adjusted. All three exchanges are thus more or less under the 

 control of the living guard-cells : the most important being the exit of water-vapour. 



Transpiration expresses the giving off of water- vapour from living plasma, as 

 opposed to the physical evaporation from free fluid or dead material. Since the only 

 available source of food salts (N,P,S, &c.) is from solution in soil-water absorbed by 

 roots, these have to be carried to the photosynthetic cells for further proteid-elabora- 

 tion, and a constant supply is required, as the Transpiration Current. The tissue 

 in which this ascends is the Xylem of the V.B. ; as the plant can only work in terms 

 of very dilute solutions, considerable quantities have to be passed through the tissues 

 in order that a small amount of salts may be retained. Transpiration is thus of 

 primary significance to all land-plants as the means of removing excess water; but 

 since too rapid desiccation will soon kill aqueous plasma, an effective mechanism of 

 regulation and control is also essential. Closing the stomata altogether would tend 

 to starve the plant. 



Stomatal Mechanism. The guard-cells absorb water from the adjacent 

 epidermal cells (or subsidiary cells) through their thin lateral wall, in virtue of their 

 greater osmotic activity. The fact that guard-cells contain chloroplasts when aqueous 

 epidermal cells do not, and still greater quantity if the latter do, suggests that they 

 retain the power of autosynthesis of osmotic material (sugars or organic acids) as 

 required. The guard-cells thus become turgid at the expense of adjacent units ; and 

 the thickening of the wall is so adjusted that they dilate most on the outer lateral wall, 

 and may be practically immovable in other regions. Hence on dilatation they become 

 convex outwards, and pull away from each other in the middle line, opening the slit 

 the wider the more turgid they become. Any stimulus to the plasmatic film, increas- 

 ing the permeability, tends to reverse the process; and water escapes back to the 

 adjacent tissues as the guard-cells become flaccid ; the pore being thus closed to the 

 original position. Excessive loss of water is an effective stimulus, and the motile 

 guard-cells as the most sensitive units feel it first. Stomata thus close on a wilting 

 plant. The effect may be imitated by plasmolysis of the guard-cells. 



Food Conduction : The mesophyll of the leaf gives sugars, starch, and proteid 

 as excess manufactured food-material, which may be conducted away to feed other 

 parts of the plant (so far parasitic on the green tissues). Sugars travel in parenchyma, 

 largely as disaccharides ; Proteids travel in sieve-tubes of Phloems : these substances 

 are less readily checked owing to lack of good test-reagents. Starch-grains stored by 

 day are readily redissolved by night, being hydrolyzed back to sugars by enzymes of 

 proteid-constitution, classed as diastase. These hydrolyze starch to dextrins, maltose, 

 dextrose in turn ; and a leaf full of starch at the end of a summer day may be quite 

 empty by next morning, and so start afresh. All parts using food-material create 

 a drain on the photosynthetic units, and under such stimulus a food-current once set 

 up may be long-continued. 



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