INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 273 



— develop a number of structural adaptions in order to retard transpiration and 

 so prevent too great an accumulation of mineral salts/ 



Plants of the far north frequently have xerophytic characters also, even 

 though they grow in very wet soil. Under these conditions they may suffer 

 from a deficiency of water, 1 for the entrance of water into the roots is dependent 

 upon certain temperature conditions; water absorption is slow when the soil is 

 cold, and if, at the same time, the atmospheric conditions produce high rates of 

 transpiration, then wilting may very easily occur, even though the roots are 

 surrounded with water. A heavy cuticle prevents the external conditions from 

 raising the rate of transpiration as much as they would if the cuticle were 

 thinner. e 



1 Kihlmann, A. Osw., Pflanzenbiologische Studien aus Russisch-Lappland. Ein Beitrag zur Kenntnis 

 der regionalen Gliederung an der polaren Waldgrenze. Helsingfors, 1890. 



f Of course this is not an explanation, and it has no bearing on the problem in hand. Plants 

 do not produce peculiar structures "in order to retard transpiration" or for any other purpose; 

 the peculiar structures result from the interaction of preexisting conditions, and the effect of 

 the presence of these structures, after they are produced, is to retard water loss. For a work- 

 ing hypothesis, it may be supposed that the high salt content of the soil retards water intake in 

 the case of these strand plants (either osmotically or by a chemical influence upon the root 

 protoplasm, such as rendering this only slowly permeable to water), and that the open exposure 

 of such plants makes the rate of water loss (transpiration) relatively high, so that the water 

 content of the tissues is maintained comparatively low. — Ed. 



g The heavy cuticle of such plants may result from low water content of the tissues (see 

 note /, just preceding). — It appears that one main reason for the dominance of plants with 

 foliar structures that retard transpiration, in bogs, and perhaps generally in the far north, 

 is the presence of toxic materials in the soil. (See note k, p. 101 .) 



This whole discussion, as given in the text, is rendered unsatisfactory by the confusion of 

 two entirely distinct problems, one physiological and the other in the realm of distributional 

 ecology. From the standpoint of physiology, we should seek the conditions (internal and 

 external) that make one plant produce xerophilous structures, etc., while another does not. 

 This involves experimental problems like that dealt with by Schlosing and by Hasselbring 

 (page 271), and like that considered by the author in reference to the experiments of Nobbe 

 and Siegert 'page 272). Without adequate measurement of the effective conditions that 

 obtain, a knowledge of these relations cannot be achieved by ordinary field observations, 

 no matter how thoroughly such observations may be subjected to subsequent attempts at 

 interpretation. 



From the standpoint of distributional ecology, on the other hand, we desire to know, first, 

 what physiological types of plants occur, and are dominant, in different habitats and geograph- 

 ical regions. As an example of this sort of knowledge we have the observed fact that thick 

 foliar cuticle is of dominant occurrence on the plants of bogs and of the far north. The 

 ecological interpretation of this observation does not have anything at all to do with the 

 physiological question as to what may be the necessary conditions for the production of thick 

 cuticle, but it does deal with the question as to what kinds of environmental complexes may 

 prevent the development of plant forms that do not produce such cuticle, at the same time 

 allowing forms that do produce thick cuticle to dominate. Given a number of plants, some 

 with and some without xerophilous foliar structures (no matter by what sets of conditions these 

 structures may have been produced or inhibited in the different cases), we observe that bog 

 habitats are characterized by the dominance of plants of the first class, and we suppose that 

 plants of the second class (without xerophilous foliar structures) are generally unable to thrive 

 in such habitats. The question then emerges, as to what are the peculiar environmental con- 

 ditions that so generally prevent the growth of the non-xerophilous forms. The generalized 

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