GERMINATION, GROWTH, TISSUE TENSION. 159 



squarely, and measure the length carefully. Then remove a strip 

 of the outer tissue and measure : it will be shorter than the whole 

 stalk. Next strip off the whole of the outer tissue and measure the 

 pith, which will be longer than the whole stalk. 



208. Transverse Tension in Stems. Cut transverse 

 slices from a fairly old portion of a woody twig, e.g. 

 Willow, and measure its circumference with a strip of 

 paper. Make a vertical slit in the stem and carefully 

 remove the outer tissue. Now try to replace the ring of 

 cortex on the wood; the ends of the ring will not meet 

 now, showing that the cortex was in a state of tension in 

 the intact stem. 



Measure the distance between the two ends of the split 

 cortex ring after replacing it, and from this calculate (1) 

 the length of the isolated ring ; (2) the tension to which 

 the cortex was subject, as a percentage of the circumfer- 

 ence of the intact stem. For instance, from a Willow 

 twig, a slice was cut with circumference 200 mm. ; the 

 distance between the cut ends of the isolated cortex ring 

 was 9 mm. ; therefore the percentage tension of the cortex 

 was 4*5. 



209. Distribution of Transverse Tensions in Stem. 



It is interesting to determine simultaneously the trans- 

 verse tension in different portions of the same stem. Cut 

 out slices from the top, middle, and base of a Sunflower 

 stem, for instance ; in each case measure the circumfer- 

 ence, then remove and measure the isolated cortex ring. 

 Note that the tension in the youngest parts is small, and 

 that it increases progressively in the older parts. 



210. Tension dependent on Water in Tissues. 



That the tension of the outer tissues in a woody stem 

 depends upon the amount of water present is easily shown. 

 Cut six slices from a Willow branch, and determine the 

 tension of the cortex in three of them at once ; determine 

 it in the other three slices after leaving them in water for 

 a day. Note that the tension increases considerably, owing 

 to the absorption of water. 



