RESPIRATION 199 



dioxide is formed which can at once be assimilated. While in ordinary 



plants the respiratory products escape from the plant, in the case of succulents 



they are retained in the leaves and carbon-dioxide arises just at the moment 



when it may be again used up. Obviously it is a matter of great difficulty to 



provide fleshy leaves with carbon-dioxide from the air. The acquisition of 



carbon-dioxide depends on the existence of wide-open stomata and abundant 



intercellular spaces, and these are features which accelerate transpiration ; but 



the succulents live under conditions which forbid copious transpiration, and 



hence they do not possess these adaptations for vigorous gaseous exchange. We 



need only refer in a word to the fact that these leaves were shown in the 



lecture on carbon assimilation also to be peculiar, since one can, on conceivable 



CO,, 

 grounds, establish in their case a value for -~ in assimilation, which varies 



very considerably from the ordinary (p. no). In the morning, at the commence- 

 ment of assimilation, the leaves give off far more oxygen than they absorb 

 carbon-dioxide, indeed they can continue to give off oxygen in an atmosphere 

 free from carbon-dioxide as long as the carbon-dioxide arising from their own 

 activity is at their disposal. 



In succulent plants, as in Mould-fungi, there are special features connected 

 with the formation of acids which are not to be explained from the chemico- 

 physiological point of view. The formation of acids has the same general 

 significance as the complete combustion of organic materials attained elsewhere, 

 but it has a subsidiary biological meaning differing widely from normal 

 respiration. There are many down-grade metabolic products of general 

 occurrence in plants which serve ecological purposes only, and it is quite likely 

 that organic acids are similarly to a certain extent, regulatory, in order to render 

 turgidity of cells possible. For example, in the formation of oxalic acid from 

 glucose a convenient medium is produced capable of inducing osmotic pressure 

 in the cell up to three times the normal. 



Let us now return to the discussion of the value of the respiratory fraction 



CO 



-~ . We have seen that variations from its typical value (= i) might be the 

 U2 



result of the formation of unusual respiratory products ; but these variations 

 may also be due to the varying constitution of the materials employed in 

 respiration. 



We have already estabhshed the fact that large quantities of fat are stored 

 in many seeds, materials which are very much poorer in oxygen than carbohy- 

 drates. When these seeds germinate the fats undergo combustion, and Saussure 

 showed long ago that during the process an absorption of oxygen took place which 

 does not correspond to the amount of carbon-dioxide given off ; the quotient 



CO 



Y) is less than unity. Bonnier and Mangin (1884) found, for example, in 



CO 

 Linum, the following values for -~ on successive days; 0-30, 0-34, 0-39, 0-40, 



0-63, 0-64. The vigorous absorption of oxygen, leading to an increase in the 



dry weight (Detmer, 1880, 335) takes place especially in the first days of 



germination ; later on, when the fats are gradually altered into carbohydrates 



CO 

 the value of -~ gradually approaches unity. He observed, for example, 



that in a seedling 3-5 cm. in length the value of the fraction was o-8i, an 

 amount as great as that seen only in plants which are beyond the seedling 

 stage (e. g. in Pinus ; Bonnier, 1884, p. 240). 



On the other hand, when fatty oils are formed from carbohydrates in ripening 

 seeds, an increase in the respiratory coefficient is naturally to be expected : 



