20 EXPERIMENTAL PLANT PHYSIOLOGY 



well at this point to examine the stems rather more 

 closely and to learn the names of the various parts 

 noticed. 



When looked at in transverse section the woody 

 stem shows three concentric zones. The centre is 

 made up of a white substance and is soft. This is 

 the pith. The middle zone is not quite so white ; it 

 is harder ; and faint radiating lines can be seen running 

 through it. This part is the wood, through Avhich the 

 solution from the soil has been found to travel. The 

 outermost layer is a very narrow one. If the stem is 

 " peeled " the whole of this band comes off. The inner 

 part of this is the bast and the cortex ; the outermost 

 part is the harlc. 



In the herbaceous stem the same parts are recog- 

 nizable. In this case the wood and bast form isolated 

 "bundles" arranged in a ring. It can easily be seen 

 that there is a small part of each bundle lying towards 

 the outside of the stem which does not stain when the 

 twig is put into the coloured solution ; this is the bast. 

 Here too the pith and cortex are connected by strands 

 of tissue ; these are termed medullary rays. 



In the monocotyledonous stem " bundles " are again 

 present, but there is no differentiation between pith and 

 cortex; for this reason all the tissue in which the 

 bundles lie is termed ground tissue. 



The Cause of the Ascent of Water in the Stem.— In 



Experiment 6 it was seen that the liquid in the thistle- 

 funnel rose to a considerable height, but it did not 

 continue to rise indefinitely, and the highest level 

 reached was but a little way when compared with the 

 height of a tall tree. 



Now it has already been stated that no full explana- 

 tion of the cause of the ascent of water through the 

 stem has yet been given, but for that very reason it 

 will be specially interesting to investigate it as far as 

 possible. 



If stem-sections are examined with a microscope, 

 the wood, through which the water travels, is seen to 



