300 THE ROOT 



root characters. In parasitic seed plants organs called 

 haustoria of very variable structure bore into the tissues 

 of the host and absorb food from it. Sometimes these 

 haustoria {Thesium, Rhinanthus) are just modified 

 branches of ordinary soil roots, which fasten on and bore 

 into the roots of other plants. In other cases (Mistletoe) 

 the parasite never has any connexion with the soil at 

 all, but the seed germinates on a tree, and its root- 

 like haustoria bore into the tissues of the tree, obtaining 

 water and mineral salts from the xylem. In other 

 cases, again (dodder), the haustoria are strands of tissue 

 (in which sieve tubes may be developed) that tap 

 the sieve tubes of the host. And finally, in the most 

 extreme cases {Rafflesia, a tropical parasite), the whole 

 body of the parasite consists simply of a branching 

 mass of strands of tissue, like the hyphse of a fungus, 

 which live entirely in the tissues of the host, occasionally 

 coming to the surface and producing immense flowers. 

 In some plants (carrot, turnip, parsnip) the main 

 taproot is greatly swollen, and acts as a food storage 

 organ through the winter in the same way as a corm 

 or tuber. The great mass of fleshy tissue is produced 

 by the cambium, and may be reckoned as parenchy- 

 matous secondary xylem and phloem, in which a few 

 strands of conducting elements are formed here and 

 there. 



PRACTICAL WORK. 



(i) Draw a single mustard seedling that has been germinated 

 in moist air (being careful to keep the root moist), and note 

 the long densely crowded root hairs, not yet full grown towards 

 the tip of the root, and the bare elongating region between the 

 root hairs and the apex. Distinguish root from hypocotyl. 

 Compare with seedlings grown in soil, and in the latter note the 

 clinging of the root hairs to soil particles. 



(2) Transfer a seedling to a slide, just covering the end of the 

 root with water under a coverslip, and examine urider the micro- 



