174 THE GROWTH OF ROOTS AND STEMS. 



hairs help greatly in this), and it must be able to resist pulling or 

 tugging strains. Hence the best arrangement for the root is to have 

 the hard tissue at the centre, forming a compact cord. 



There is great diversity in the arrangement of the hard tissue 

 (sclerenchyma), which is, next to the wood of the vascular bundles, 

 the most important supporting tissue or "stereom" (hard-tissue) of 

 the majority of erect herbaceous stems. Sclerenchyma-fibres, which 

 occur among the vessels in the wood as well as in separate bands or 

 strands outside of the bundles, are dead cells (like those which make 

 up the wood- vessels) ; whereas collenchy ma- cells are living, and can 

 carry on assimilation, and are also capable of growth, the walls not 

 being hard and woody, as in sclerenchyma. In some plants e.g. 

 Grasses, Sedges, Horse-tails the epidermis contains flinty substance 

 (silica) which helps in giving rigidity. 



Cut cross-sections of various herbaceous stems, stain with aniline 

 chloride, and examine with lens and microscope, giving special atten- 

 tion to the arrangement of the supporting tissue in the vascular bundles 

 (wood), sclerenohyma, and collenchyma. Of special interest are ridged 

 and winged stems (Gorse, Broom, Bilberry) and the stems of Grasses, 

 Rushes, and Sedges. 



203. Is the "Skeleton" alone able to keep the Young Shoot 

 erect? What happens if you pull up a whole seedling, e.g. Bean or 

 Sunflower, or cut off the shoot, and let it lie on the table ? Why does 

 it turn limp ? Weigh a shoot after cutting it off, then weigh it after it 

 has been allowed to wilt : what has the shoot lost ? How can you make 

 the limp shoot become stiff again ? Why is it useless to try to restore 

 a shoot that has been allowed to lie too long and has become dry ? 



204. How are Limpness and Firmness produced? 



It is due to osmosis that the shoot turns limp on losing water 

 and recovers its firmness on being placed in water. 

 * (a) Cut off a seedling's shoot and put it into 5 per cent, salt solution ; 

 it becomes limp after a time (why ?). Take the limp shoot out of the 

 solution, wash it under a tap, and set it in water ; what change occurs, 

 and why ? 



(b) Is it necessary to put the whole shoot in the salt solution ? 

 Place one fresh shoot with its cut end and another with its free leafy 

 end dipping into the solution and afterwards into water. 



205. Plasmolysis and Turgidity. The shoots used in 

 these experiments are not killed unless the salt solution is 

 made too strong or they are left too long in it. Prove this by- 

 pulling up whole seedlings and re-planting them, after making 

 them turn limp, or flaccid, in the salt solution. 



(a) Cut sections of a piece of fresh beetroot ; mount sections 

 separately in (1) water; (2) salt solution; (3) alcohol. In (1) notice 



