324 PLANT PHYSIOLOGY 



an obstacle to growth, with the result that there is not the re- 

 straining force imposed upon its growth by the environment as 

 in the case of the root. The region of stem growth varies with 

 the plant, but often extends for several internodes (5-8) back 

 from the tip, covering a region many centimeters long. 



The tissues which make up an organ do not all grow at the 

 same rate. As a result of this differential growth, there thus devel- 

 ops within the organ tissue tensions; some tissues are stretched 

 and others are compressed. These tissue tensions may occur either 

 longitudinally or transversely. The former may be easily seen if 

 a stem of dandelion, celery petiole, or other growing stem is cut 

 lengthwise. Immediately after the two halves are separated, the 

 stem will curve so that the outer surface becomes concave. This 

 shows that the pith was compressed and the cortex stretched 

 under the conditions present in the growing stem; but as soon as 

 this tension is released by cutting, the pith then expands while 

 the cortex contracts, with the resultant curvature. If the stem 

 is placed in water, the pith cells take up more water than the 

 cortex so that the expansion of the former becomes still more 

 pronounced and the stem may become much coiled and twisted. 



Transverse tensions may be seen best in the cortex of woody 

 plants. Since the wood increases in amount more rapidly than 

 the cortex, which does not split off very rapidly, the wood is 

 compressed by the bark, which is, in turn, stretched by the pres- 

 sure from within. If a ring of bark is removed as in girdling and 

 is then replaced in its original position, the two ends fail to meet 

 owing to the contraction of the band of bark after its removal. 



Leaves grow more evenly than stems or roots throughout their 

 entire surface with a slight increase at the basal region over the 

 apical one. As they unfold, the meristematic region is nearer the 

 base, where the leaf is connected with the stem, and it is in this 

 region consequently where the greatest growth occurs (Fig. 22). 



Conditions Influencing Growth. — The factors which affect the 

 growth of plants are either external or internal. Among the 

 internal conditions, age and vitality are the most important. The 

 connection of the grand period of growth with the age, discussed 

 in the previous paragraphs, is an illustration of the weight of 

 this factor. 



Heredity is another factor which cannot be escaped. A sun- 

 flower can never be made to grow as large as an oak even with the 



