THE GROWTH OF ROOTS AND STEMS. 175 



the layer of protoplasm ("primordial utricle") lining the cell- wall, 

 and the red sap filling the cavity (vacuole) of the cell ; some of the 

 cells will be cut open, allowing the sap to escape. In (2) notice 

 that the primordial utricle contracts from the cell-wall, but still 

 contains the red sap, which does not escape from the living cell. In 

 (3) notice that the red sap diffuses out of the cells, which have been 

 killed by the alcohol. 



* (6) If you have no microscope, the following simple experiment will 

 give you some idea of turgidity. Cut a piece, about 3 by 1 by 1 in. , 

 out of a fresh beetroot, and notice the way in which it resists 

 being bent between your thumb and forefinger applied to its ends. 

 Then place it in salt solution, and notice that it bends much more 

 easily (why ?) ; then place it in water until it becomes stiff again (by 

 the cells becoming turgid). Then plade the piece of root in boiling 

 water for a few minutes, rinse it in cold water and see whether it can 

 be made turgid again. 



206. Tension Caused by Turgidity. Are all the cells of 

 a growing plant equally turgid ? Have you ever noticed that 

 the cut ends of flower-stalks (e.g. Daffodil or Tulip) dipping 

 into water become split in a peculiar way, and that the split 

 parts curve outwards and upwards ? 



* (a) Make short slits in the cut end of a seedling stem (or the flower- 

 ing-stalk of a Daffodil, Tulip, Bluebell, etc.) and set it in water. The 

 curling of the slit parts is evidently due to the inner cells absorbing 

 water more rapidly than the outer ones. 



In addition to the three supporting or " skeletal " tissues 

 wood-vessels, sclerenchyma, collenchyma the ordinary 

 thin- walled tissue (parenchyma) plays an important part in 

 maintaining the rigidity of herbaceous stems as well as of 

 petioles, leaf -blades, and flower- stalks, by the turgidity of its 

 cells. In a herbaceous stem the pith has a strong tendency 

 to elongate, but this is hindered by the outer tissue, and the 

 state of strain thus set up tends to keep the stem rigid and 

 erect. The outer tissue is on the stretch, tending to shorten, 

 while the inner tissue is under compression. 



If a strip of Sunflower stem 50 cms. long is cut out and 

 the soft tissue (pith) separated from the outer tissue, the 

 former suddenly lengthens to over 60 cms., while the latter 

 shrinks to 45 cms. In the same way, if we cut a turgescent 

 stem, say of a young Elder twig, flat on two opposite sides 

 and then bisect it, each half bends outwards ; the pith and 

 cortex assume their proper lengths, the former expanding 

 and the latter contracting. 



