370 PHYSIOLOGY OF GROWTH. 



epidermis, collenchyma, green tissue, and pith parenchyma follow 

 each other in succession from without inwards ; in the ground 

 tissue lie the fibro-vascular bundles. If we split a young flower- 

 stalk longitudinally, and lay the pieces in 

 water, they rapidly roll up into a spiral 

 under the eye of the observer, the pith side 

 being convex (see Fig. 118). The medul- 

 lary tissue rapidly takes up large quantities 

 of water, its cells lengthen, and the structure 

 consequently curls up spirally. Two inter- 

 nodes, as nearly as possible similar to each 

 FIG. us. Flower scape other, are cut oft and examined in the 



iSTTlflSfc manner abOTe described as regards their 

 which has coiled in con. tensions. One is investigated immediately 



sequence of absorption of ^^ being ^ Qff . ^ ^^ after ifc hag 



become somewhat limp through being left 



in the air. We shall find the difference in length between the 

 isolated epidermis and the isolated pith greater for the first than 

 for the second, which again indicates that the amount of water 

 in the tissues is of great significance in determining the magni- 

 tude of the tensions occurring in plant structures. 1 



1 See Kraus, Botan. Zeitung, 1867. Further information of importance in 

 disscusing the subject of tension in tissues will be found in my Lehrbuch der 

 Pfianzenphysiologie, 1883, p. 229. 



150. Transverse Tension. 



To determine the existence of transverse tension in any part of 

 a stem structure, we cut out a transverse slice at this place and 

 measure its circumference with a strip of paper, then break the 

 continuity of the peripheral tissues by a perpendicular radial cut 

 and strip off the whole cortex. The isolated ring of cortex is now 

 replaced in its original position without stretching. It is found, 

 however, that the cut surfaces no longer meet together, whence 

 it follows that the cortex must, in the intact structure, be passively 

 or negatively stretched (see Fig. 119). If we measure the distance 

 between the two ends of the divided ring of cortex after it has 

 been replaced, and subtract the result from the length of the 

 circumference of the intact transverse slice, we obtain a number 

 which expresses the length of the ring of cortex after isolation. 



