INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 307 



those conditions. Although the internal conditions that influence the value of this 

 moisture ratio (2 and 4, above) are capable of great internal adjustment, yet this adjust- 

 ment is limited for each plant form; consequently some forms require a moist climate, 

 others a dry one, etc. Plants that thrive in dry periods are characterized by structures 

 that facilitate water absorption and hinder transpiration. Within the limits of its 

 capacity for internal adjustment, the same plant form may develop under widely 

 different moisture conditions. With relatively humid conditions, the internodes are 

 generally long and the leaf blades are extensive, the cuticle is thin, and there is not 

 much woody tissue, etc. With relatively arid conditions, the internodes are generally 

 shorter and the leaves are smaller, the cuticle is thicker, and there is a greater develop- 

 ment of woody tissue. Arid conditions often promote the formation of thorns and 

 spines — also the development of succulence (as is usual for cacti). In some plants the 

 morphological characters are so much influenced by the moisture conditions of the 

 surroundings that two individuals, one grown in an arid and the other in a humid 

 climate, often appear to be quite different species. Some plants that develop partly 

 under water and then extend into the air produce very different structures in the two 

 environments, and this difference seems to be largely related to moisture conditions. 



As to the manner in which external moisture conditions influence plant growth and 

 development, a few considerations may be mentioned. Growth by enlargment can 

 not occur in cells that are not turgid, and the rate and kind of growth that occur depend 

 upon the degree of turgidity present. The turgidity of every cell is inversely propor- 

 tional (other conditions being constant) to the resistance offered to water absorption or 

 to the forces tending to remove water from the cell. The relation of the rate of water 

 supply to that of water loss determines whether turgidity shall increase, decrease, or 

 be maintained. 



If the environmental moisture conditions remain constant, on the other hand, the 

 turgidity (and growth) of any cell may be altered by internal changes, such as alter- 

 ations in the permeability of wall or protoplasm to water, or alterations in the osmotic 

 and imbibitional attraction for water, exerted by the cell contents. Such internal 

 changes are promoted by alterations in the amounts of various dissolved substances 

 within the cell, such as acids, salts, etc. 



The transpiration rate largely determines the rate of entrance and transport of 

 dissolved materials from the soil (when the soil moisture supply is adequate and the 

 cell membranes are permeable to these solutions) ; the chemical content of any tissue 

 is therefore partially controlled by the water conditions of the surroundings, and the 

 chemical content of a cell markedly influences its growth. The kinds and amounts 

 of substances dissolved in the soil water exert potent influences on the growth of 

 ordinary plants. 



The roots of many plants are hydrotropic. When exposed to unlike moisture 

 conditions on opposite sides, the root elongates more rapidly on the drier side, thus 

 producing a bending of the root away from the drier environment. This response to 

 one-sided moisture conditions is due to hydrotropism. 



5. Influence of Light on Growth and Configuration. — Light conditions are very 

 important in determining not only the rate but also the kind and extent of growth in 

 ordinary plants. Stems generally elongate less rapidly by day than by night, and this 

 seems to indicate that ordinary daylight retards stem elongation. Plant stems in 

 continuous darkness elongate more rapidly and produce longer internodes than do 

 those in light or with the natural day-night fluctuation, but leaves generally remain 

 very small in maintained darkness and expand to their regular size only when light is 



