62 GROWTH AND WORK OF PLANTS 



at their adjacent ends dissolve and thus make long tubes or 

 vessels of the connecting cells. These vessels marked with 

 rings, spirals, pits, etc., can often be found in the same bundle. 

 In some plants the spiral thickenings on the vessels are so regular 

 and so strong in contrast with the thinner portions of the wall and 

 the other tissues that when the stem is broken the spiral will un- 

 coil in long, delicate, cobweb threads. This is shown in a re- 

 markable way in the flower stems and petioles of the Indian 

 lotus (figs. 56-58). The rind of the stem is cut with a knife and 

 the stem then broken. The uncoiled spiral is strong enough to 

 support by suspension a piece of the stem 2 cm. to 4 cm. long, 

 and so delicate that one cannot see the means of support at a 

 little distance unless held before a black object or the light. 

 Examined under the microscope the beautiful spiral markings 

 can be seen. 



STRUCTURE OF THE STEM OF DICOTYLEDONS. 



02. Gross structure of the stem of an annual. A cross 

 section of the stem of a dicotyledon shows a very different 

 structure (fig. 54, A). Leafy shoots of dicotyledonous stems like 

 the garden balsam or touch-me-not (Impatiens), bean, sun- 

 flower, etc., may be placed with the cut ends in red ink or a 

 solution of a red anilin dye. After several hours the loss of 

 water from the leaf will draw the colored liquid up through the 

 vascular ducts. This will stain the bundles, and also the veins 

 of the leaves in many cases, so that the color is easily seen through 

 the thin overlapping tissues. If shoots with white flowers are 

 also placed in the dye the white petals will become stained. If 

 the stems are now cut across the position of the bundles can 

 be seen. Instead of being scattered without order through the 

 stem they are arranged in a ring (with spaces between them) 

 about midway between the center of the stem and the outside. 

 The central part of the stem is the pith. The portion outside 

 of the ring of bundles is the cortex. The radiating strands 

 of tissue lying between the bundles and connecting the pith 

 with the cortex are the medullary rays. 



