342 HOW CROPS GROW. 
the point of their issue. 
Girdling a fruit - bearing 
branch of the vine 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. x 
The accompanying illustration, 
fig. 66, represents the base of a cut- 
ting from an exogenous stem (pear 
or currant) girdled at Band kept for 
some days immersed in water to the 
depth indicated by the line Z. The S 
first manifestation of growth is the j 
formation of a protuberance at the ik 
lower edge of the bark, which is Af 
known to gardeners as 4 callous, C. f 
This is an extension of the cellular AR 
tissue. From the callous shortly 
appear rootlets, 2, which originate 
from the vascular tissue. Rootlets 
also break from the stem above the 
callous and also above the water, if 
the air be moist. They appear like- 
wise, though in less number, below 
the girdled place. 
Nearly all the organic sub- 
stances (carbohydrates, atl- 
buminoids, lignin, etc.,) that Fig. 66, 
