Grafts et al. 



192 — 



Water in Plants 



explanations appear. The most obvious would involve a rhythmic fluctua- 

 tion in stomatal movement in the dark, such as has been observed in some 

 plants. Sayre (1926) found a diurnal opening of stomata in the dark for 

 two days. After this length of time the degree of opening was reduced. 

 The curves presented by Montermoso and Davis appear to show a decline 

 after about this same period of time. Tagawa ( 1936) observed continued 

 stomatal movements in beans for several days when kept in constant dark- 

 ness or when exposed to constant light. The second possible explanation 

 would involve a diurnal fluctuation in root resistance. Such changes in 

 root pressure have been discussed in Chapter IX. 



.30 



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M/}. 



/^oon 



Mn. 



A/oon 



Mn. 



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Fig. 54. — Diurnal fluctuations in water loss from rooted Coleus leaves under controlled condi- 

 tions. The leaves were in the dark during the above tests and temperature and humidity were con- 

 stant. The solid line represents plants preconditioned under normal light-dark conditions, the 

 dashed line plants preconditioned under reversed light-dark cycle. From Montermoso and Davis 

 (1942). 



Internal Factors Affecting Transpiration: — The anatomical and 

 physico-chemical differences among species, and between individuals of a 

 single species causing variation in transpiration rate are difficult to differ- 

 entiate and evaluate. However, they have received considerable study. It 

 is convenient to group them into two sets (a) those affecting the diffusion 

 pressure of water in the walls directly, and (&) those influencing the rate 

 of removal of water from the sub-stomatal chambers. In the first group 

 are i) wall structure, 2) physical resistance to water movement (friction), 

 and 5) osmotic properties of cells. In the second group are 1) internal 

 exposed surface, 2) stomatal aperture, 5) cuticle, and 4) morphological 

 modifications. 



Wall Structure: — The cell walls of plants are made up largely of carbohydrates 

 and polyuronides. The organic portions of these compounds have the generalized ap- 

 proximate formula Cn H[2i.-2: Ocn-i: where n has a value of 5 or 6 or some simple 

 multiple thereof. From this formula it is evident that, of the organic portion of the 

 molecules, around 25 per cent of the atoms are oxygen. And whereas each of these 

 oxygens has a covalence of 2 that is ordinarily satisfied in the molecular constitution 

 of the compound, an equal number of secondary or residual valences exist, each of 

 which is capable of holding one water molecule through a hydrogen bond. Further- 

 more, the cellulose and uronide chains making up the walls are so spaced in the micelles 

 or fibrils that they form a microcapillary matrix capable of holding much water by 

 microcapillary forces. 



When the hydrostatic pressure of the water in the xylem approaches atmospheric 



