128 GENERAL BOTANY 



branches, and the combined effect of their countless 

 root hairs made roots stick very tight in the ground. 

 Some roots, however, not only keep the shoot firm, 

 but actually drag it deeper into the soil. This is 

 especially the case with some monocots that have 

 horizontal rhizomes, e.g. CANNA. If the soil from 

 above the rhizome be scraped away, so that the latter 

 is not so deeply buried, the roots contract and drag it 

 further downwards. 



There are some plants too, whose roots have no root 

 hairs. They are mostly water plants, whose roots 

 being always in water do not require hairs and many 

 land plants which normally have root hairs, do not if 

 grown in water. 



4. SPECIAL FORM OF ROOTS. We ha\e already 

 referred to the Sweet-potato as being a root-tuber. 

 The garden DAHLIA is another plant whose roots 

 become swollen and tuberous. In the common country 

 Radish, it is the tap-root that becomes swollen, and 

 the branch roots, which arise in two vertical lines, 

 on either side of it, are quite thin and small. (In 

 the English radish the tuber is formed from part of 

 the hypocotyl). These tuberous roots, have the same 

 importance as rhizomes, stem-tubers and corms car- 

 bonaceous food-material, mostly in the form of starch, 

 is stored in them for the use of the next year's shoot 

 and its seeds. 



Very peculiar roots occur on some plants which 

 grow not in the ground but on trees. These epiphytes, 

 as they are called, cling to trees by small roots 

 which like those of root-climbers are strongly aphelio- 

 tropic, and making their way into the crevices of 



