220 BOTANY 



death at low temperatures results, not from the direct influence of the cold, but 

 from the insufficient absorption of water, the roots being unable to take from the 

 cold or frozen soil enough water to make good the transpiration from the sub-aerial 

 organs. 



6. Death does not necessarily at once result when the minimum 

 or maximum for external factors is overstepped. The organism has 

 frequently passed into a condition of LATENT LIFE, and this may also 

 come about from internal causes. It is often difficult to decide from 

 inspection whether an organism is in the condition of ACTIVE LIFE, of 

 LA.TENT LIFE, or of DEATH. Latent life has this in common with 

 death, that all vital activities are arrested ; but while active life can 

 be resumed from latent life, this is impossible when the organism 

 is dead. 



Many resting stages of plants, such as seeds and spores, pass into the state of 

 latent life. They are then as a rule far more resistant to desiccation, heat, and 

 cold than organs in an active condition. Thus spores of Bacteria can bear a moist 

 heat of 100 C. and more, and the same holds for some seeds, such as those of species 

 of Medicago. On the other hand, spores and seeds in the dry condition resist a low 

 temperature even of - 253 C. ( 5 ) 



SECTION I 



METABOLISM () 



I. The Chemical Composition of the Plant ( 7 ) 



Any consideration of the metabolic changes in the plant requires 

 a knowledge of its chemical composition. This is studied by chemical 

 methods. 



Water and Dry Substance. Some insight into the composition of 

 the plant can be obtained without special means of investigation. 

 Every one who has dried plants for a herbarium knows that the plant 

 consists of water and dry substance. He also knows how the removal 

 of the water influences such fundamental physical properties of the 

 plant as its rigidity and elasticity. By means of weighing it is 

 easy to show how large is the proportion of water in the total weight 

 of the plant. For this purpose it is not sufficient to expose the plant 

 to the air, for when air-dried it still retains a considerable proportion 

 of water, which must be removed by drying in a desiccator or at a 

 temperature of over 100 C. It can thus be ascertained that the 

 proportion of water is very considerable ; in woody parts some 50 per 

 cent, in juicy herbs 70-80 per cent, in succulent plants and fruits 

 85-95 percent, and in aquatic plants, especially Algae, 95-98 per cent, 

 of the weight of the plant consists of water. 



