342 



HOW CROPS GROW. 



the point of their issue. 

 Girdling a fruit - bearing 

 branch of the vino near its 

 junction with the older wood 

 has the effect of greatly en- 

 larging the grapes. It is 

 well known that a wide 

 wound made on the stem of a 

 tree heals up by the formation 

 of new wood, and commonly 

 the growth is most rapid and 

 abundant above the cut. 

 From these facts it was con- 

 cluded that sap descends in 

 the bark, and, not being able 

 to pass below a wound, leads 

 to the organization of new 

 roots or wood just above it. 



The accompanying illustration, 

 fig. 66, represents the base of a cut- 

 ting from an exogenous stem (pear 

 or currant) girdled at B and kept for 

 some days immersed in water to the 

 depth indicated by the line L. The 

 first manifestation of growth is the 

 formation of a protuberance at the 

 lower edge of the bark, which is 

 known to gardeners as a callous, C. 

 This is an extension of the cellular 

 tissue. From the callous shortly 

 appear rootlets, It, which originate 

 from the vascular tissue. Eootlets 

 also break from the stem above the 

 callous and also above the water, if 

 the air be moist. They appear like- 

 wise, though ;n less number, below 

 the girdled place. 



Nearly all the organic sub- 

 stances (carbohydrates, al- 

 buminoids, lignin, etc.,) that 



Fig. 66. 



