102 PHYSIOLOGICAL CONDITIONS AND FACTORS [CH. 



Let us now consider how this explanation fits various cases. 



(1) In a normal plant with green leaves, coloured flowers and tinged 

 stems and petioles, the chromogen is synthesised from sugars in the 

 leaves and translocated away when formed. If the plant is kept in 

 the dark or shade, photosynthesis stops or is lessened, the supply of 

 chromogen falls below normal, and the flower-colour may be pale and 

 the stems and petioles green. Conversely, great photosynthetic 

 activity produces a plentiful supply of chromogen which results in 

 rich flower-colour and appearance of pigment in the vegetative organs. 

 (In addition, light itself may directly increase pigment formation.) 

 The intense colours in the flowers and the development of anthocyanin 

 in the vegetative parts of High Alpine plants may be explained by 

 strong insolation, stunted growth employing little material, and slow 

 translocation due to low night temperature 1 . The power of some 



1 Exception will probably be taken to this statement on the ground that many High 

 Alpine plants can be grown in lowland regions and their flowers do not then show any 

 perceptible loss of colour, whereas if the statement were true, we might expect to find 

 considerable diminution of colour under these conditions. As a matter of actual fact, 

 on the basis of observation, this criticism is not altogether valid, for, as we have previously 

 stated, Gaston Bonnier (328). Kerner (398) and others have shown that in many cases the 

 flowers of plants growing at high altitudes and in high latitudes have a more intense colour 

 than those of individuals of the same species grown either in the plains or in lower latitudes. 

 There is also a more general aspect of the question, which may be outlined as follows. 

 Every plant is the expression of a chemical (or, fundamentally, physical) entity, and this 

 expression can only fluctuate within limits on account of the defmiteness of the chemical 

 (or physical) constitution underlying it. Broadly speaking these chemical (or physical) 

 entities are adapted to their habitats, that is, they are only able to exist under those con- 

 ditions in which the chemical reactions (or physical processes) essential to their existence 

 can take place. Sometimes a plant, for example species of Opuntia, will live and flourish 

 to a certain extent in a habitat to which it is not adapted, such as the temperate zones. 

 The conditions for its metabolic processes do not in this case lie beyond the limits of such a 

 climate. But the expression of its entity, of necessity, remains the same. We may assume 

 that, for Opuntia, variation in the direction of the characters of plants of the temperate zones 

 does not occur, since such changes would involve chemical reactions (or physical processes) 

 which are outside the sphere of its constitution. Hence it remains a typical species of 

 Opuntia. Other examples no doubt could be selected where there were greater fluctuations 

 on change of habitat, but these again would depend either on the wideness of the sphere 

 of chemical (or physical) activity of the particular plant, or on the relation of the particular 

 fluctuation to the environment. The same line of argument we have applied to Opuntia 

 can be applied to the members of any plant formation, and among them, to High Alpine 

 plants. These are on the whole adapted to their environment, and intense flower-colour 

 is one of the expressions of the entity of this particular type of plant. Transferred to 

 lowland regions, such a plant is still able to grow and flourish, and of necessity it retains 

 its entity. Yet fluctuations in colour intensity of the flowers and in the amount of 

 pigment in the vegetative organs are, within limits, the kind of variation we should suppose 

 possible, though not necessarily inevitable ; for such fluctuations depend merely on the 



