86 ANATOMY AND PHYSIOLOGY OE 



exist for months in tHs sickly condition, but they cannot bear it 

 permanently. 



On the other hand, when the respiration connected -with the 

 consumption of carbonic acid is stimulated by affording to the 

 plant, while exposed to light, an unusual quantity of carbonic acid, 

 its nutrition is rendered more active. Even when nothing but 

 water and carbonic acid are given, they are able to increase their 

 organic substance, and the weight of this increase amounts to 

 somethinpf like double that of the carbon which is contained in 

 the absorbed carbonic acid. 



Ohserv. In an experiment of Saussure's, little plants of Vinca appro- 

 priated 217 milligrammes of carbon from the carbonic acid absorbed, and 

 their organic substance was increased about 531 milhgrammes ; two 

 plants oi Mentha sativa consumed 159 milligrammes of carbon and in- 

 creased m weight about 318 milligrammes ('^JKechercheSp^ 226). 



When the respiration of plants connected with absorption of oxy- 

 gen and formation of carbonic acid, is interrupted by placing the 

 entire plant in air containing no oxygen, for example in nitrogen, 

 or by placing the plants under the air-pump, all their functions 

 at once become paralyzed. The unfolding of the leaves and buds 

 is cheeked and they rot, the leaves no longer turn towards the light ; 

 they no longer exhibit the alternate movements of waking and 

 sleeping; sensitive leaves lose their irritability (Dutrochet, "Me- 

 TnoiTes'' i> 361, 483) ; even single organs cut off fiom air decay while 

 the rest live on : for instance, roots which are covered too deeply 

 with earth. Plants die particularly soon when kept in air devoid of 

 oxygen, in the dark; for example, a Cactus — a plant generally so 

 obstinately retentive of vitality — died in five days (Saussure, I. c. 

 87). Plants bear being placed in such an atmosphere better when 

 they are exposed to the alternations of day and night, since they 

 exhale a small quantity of oxygen from their own substance by 

 day, and from this form carbonic acid at night, which is again 

 consumed ty day. Plants are capable of holding out in this way 

 a long time, although certainly in a very miserable way and with- 

 out manifesting growth ; but if the small quantity of oxygen 

 which they form is removed by sulphur and ii^on filings, or the 

 carbonic acid by lime water, they are unable to form these gases 

 a second time, and die. 



It is clear, from the preceding facts, that the respiration of 

 green coloured parts during the action of light is related to the 

 nutrient processes of the plant, since these become abnormal when 

 the function is interrupted, but yet the plant can maintain its 

 existence a long time under these conditions. But that which 

 occurs in common to all parts, and which consists of absorption of 

 oxygen and exhalation of carbonic acid, stands in immediate rela- 

 tion to the life of the plant. If the chemical process, which goes 

 on unceasingly in all the organs of plants, through the action of 



