3i6 



MET A MORPHOSIS 



increase in amount we at length reach certain quantities of each which produce 

 an optimum effect, and when these amounts are exceeded injurious results 

 follow, due either to osmotic or to chemical influences, and resulting in death 

 if a certain maximum be exceeded. 



Amongst the substances which are essential to most plants oxygen is of 

 special importance. It is not a ' food-stuff ' in the ordinary sense of the term, 

 since it is associated, not with constructive, but with destructive metabolism, 

 e. g. with respiration. Growth is affected in a most marked manner by the 

 degree of concentration of oxygen present. If care be taken that the air-pres- 

 sure as a whole remains unaltered, decrease in oxygen pressure induces an 

 acceleration of growth, so that the normal amount of oxygen in the air may be 

 considered as supra-optimal so far as growth is concerned. In many cases, 

 however, by increasing the partial pressure of oxygen 

 an increase in the rate of growth may be observed, so 

 that there would appear to be two optima as regards 

 concentration of oxygen. Possibly this may be an in- 

 direct result of experimental conditions. In all cases, 

 however, we are able to increase or decrease the 

 amount of oxygen present in the air to such a degree 

 as to retard growth and finally bring about death 

 (maximum and minimum oxygen pressures). After 

 what we have already learned as to the oxygen- 

 requirements of different plants, the specific differences 

 in minima and maxima of that gas are easily under- 

 stood. A concentration which is subminimal to an 

 ordinary aerophilous plant may be supramaximal to 

 aerophobous ones such as anaerobes [compare PERODKO 

 (1904)]. As transitional forms between these two ex- 

 tremes, the sulphur Bacteria are of special interest, 

 because they have a very low oxygen-optimum, although 

 oxygen is absolutely essential to them. While typical 

 anaerobes require as a condition of growth a very low 

 partial pressure of oxygen, genuine aerobes as a rule do 

 not exhibit growth when the conditions are such as 

 to induce intra-molecular respiration (WiELER, 1883. 

 and 1901), or at most they show only a very insignifi- 

 cant increase in length (NABOKICH, 1901-2). 



As already noted, many of the substances essential 

 to plant life act injuriously when certain definite con- 

 centrations are reached, and if the injury be referable 

 to the chemical action of the substance we may term 

 it a ' poison '. Many metabolic products found in plants 



are in this sense poisonous both to the plants which produce them and to 

 others as well. [NIKITINSKY (1904) has also discovered metabolic sub- 

 stances in Mould-fungi which accelerate growth in these plants.] Plants are 

 in general capable of resisting the action of their own metabolic products, but 

 only if they occur in limited amount. Retardation of development results 

 in the long run from the presence in excess of alcohol or acids produced 

 during fermentation, and so also in the higher plants the products appear- 

 ing in them may act injuriously if they be not used up, e.g. carbon-dioxide,, 

 or be not made insoluble and hence innocuous, as, e.g., when oxalic acid 

 unites with calcium. There are also substances, however, which never occur 

 in plants and with which they do not come into contact in nature, which are 

 virulent poisons, retarding growth even in very dilute solutions. It is unneces- 

 sary to enumerate these poisons here ; we need only note that many substances 



Fig. 93. Seedling rose 

 grown in the Strassburg Bo- 

 tanic Garden, which after 

 forming a few leaves pro- 

 ceededto form flowers in the 

 first year of growth (nat. size). 



