54 PLANT PHYSIOLOGY AND ECOLOGY 



of vessels, and short wood-parenchyma cells, usually filled with 

 starch. Arising from the cambium and running radially through 

 the bast and wood are the medullary rays. These consist of rows 

 of parenchyma cells elongated radially. They serve as pathways 

 for food solutions passing to and from the storage-cells of the pith, 

 and as storage-cells for starch. 



69. Functions of the stem. The stem is primarily an organ 

 for support. Its most important secondary function is the trans- 

 port of the water taken up by the root. The structural features 

 peculiar to the stem are to be found in the arrangement and com- 

 position of the fibrovascular bundles. In the monocotyledons 

 the need for increased support is met by the formation of new 

 bundles. Dicot3dedons can attain the same end by increasing 

 either the number of bundles or the size of each bundle. Woody 

 herbs, shrubs, and trees unite both methods, first increasing the 

 number of bundles as well as the size, and later, after the ring 

 becomes closed, increasing the size alone. In herbaceous plants 

 practically all of the elements of the xylem play a part in the 

 transport of water, but in woody plants the thick-walled fibers 

 have little or no part in this function. 



Experiment 13, Structure of stems. Cut thin cross- and longi- 

 sections of sunflower, corn, and ash, the first two preferably by means 

 of the i^araffin method. Make a schematic drawing of the cross-section 

 showing the fibrovascular system. Draw a segment from the cross- and 

 longisection of each stem under the high power, and indicate the various 

 regions and elements. 



70. The upward movement of the water. When the water has 

 been brought by diffusion to the radial bundle of the root, osmosis 

 ceases. This is due to the fact that the elements of the bundle, 

 fibers, tracheids, and vessels, are dead cells which lack protoplasm 

 and for the most part have rigid and thickened walls. Under 

 these conditions, osmosis and consequent diffusion become im- 

 possible, and imbibition and infiltration take their place. These 

 in themselves are slow processes, and require to be emphasized 

 in order that water may enter the bundle more rapidly. This 

 emphasis is furnished by the cumulative action of the turgid cells 

 of the cortical parenchyma. The constant absorption of water 

 by the root-hairs, and its diffusion through the parenchyma, put 

 an increasing strain upon the elastic wall of each cell and upon 



