142 A TEXTBOOK OF GENERAL BOTANY 
of sugar is obtained from this tree by boring into the wood in 
the spring. A similar phenomenon is observed in the exudation 
of sweet sap from tapped inflorescence stalks of palms, such as 
the coconut (Figs. 137, 188). Such sap is frequently used for 
manufacturing alcohol or sugar. 
Water. The movement of water upward in the stem takes 
place in the vessels. This can be very clearly shown by cutting 
off a stem and placing the lower end in a colored solution. If 
the stem is cut across, it will be found, after sufficient time has 
elapsed, that the colored solution is inside the vessels. Water 
enters the outer cells of the roots, passes through the cortex 
into the stele, and enters the vessels. It then passes through 
the vessels of the root to those of the stem, and through these to 
the leaves, flowers, and fruits. The loss of water by transpira- 
tion from the chlorenchyma cells tends to increase the concentra- 
tion of the contents of these cells. By osmosis and hydration 
of colloids the chlorenchyma cells draw water from the vessels, — 
and so replace that which is lost through transpiration. 
SECONDARY THICKENING 
Formation of cambium. The vascular bundles of dicotyledons 
contain a single layer of cambium cells, which separates the xylem 
from the phloém (Fig. 113). While the other cells formed by 
the division of the meristem in the tip of the stem are being 
differentiated into permanent tissue this layer of cambium in 
the bundles remains meristematic. When the primary xylem 
and the primary phloém are first differentiated, there is no cam- 
bium across the pith rays to connect the edges of the cambium 
in the bundles. After the differentiation of the primary tissues 
the parenchyma cells of the pith rays, which lie between the 
edges of cambium in the bundles, divide in a plane at right 
angles to the radial direction of the rays and form a layer of 
cambium across the pith rays. The beginning of this process 
is shown in Fig. 115. The newly formed cambium connects the 
cambium in the bundles, and this results in the formation of 


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