188 



THE GROWTH OF BOOTS AND STEMS. 



really includes part of the stem in addition to root proper, 

 and in Turnip and Radish the whole of the swollen part 

 corresponds to the hypocotyl of the seedling. 



In perennial plants the roots may be annual, as in Lilies 

 and other plants with bulbs or rhizomes ; or biennial, as in 

 Dahlias ; or perennial, as in shrubs and trees. Roots that 

 are annual i.e. perishing the same year in which they are 

 formed contain no reserve food ; but roots that last two or 

 more years nearly always contain more or less food for the 

 following year's growth. 



Adventitious roots may be used for the storage of food, as 

 in the tuberous roots found in many British Orchids. The 

 tubers may be either palmate or globular, and they arise 

 adventitiously from buds which appear at the base of the 

 present year's stem. Next year a new stem develops from 

 this bud at the expense of the nutriment stored up in the 

 tuber. In the case of the Dahlia and Paeony, adventitious 

 tuberous roots grow out from the base of the stem (Fig. 62), 

 and these are put to a similar use as storage houses for 

 reserve-materials. That these bodies are really roots is shown 

 by their development as irregular 

 endogenous outgrowths at the base 

 of the stem, which do not arise in 

 the axils of leaves and are at first 

 covered by a root-cap. 



(a) In Radish seedlings, make sketches 

 of young plants of different ages, showing 

 the cotyledons and the red hypocotyl, 

 the thickening of the hypocotyl, and the 

 fully formed swelling. Cut thin cross- 

 sections of the "root" (hypocotyl) of a 

 young seedling, and notice the very 

 marked circular transparent zone of 

 growing tissue (cambium) ; test with 

 iodine and notice the starch present in 

 most of the cells except those of the 

 cambium. The cambium consists of 

 narrow oblong cells, arranged in the 

 radial rows, filled with protoplasm, and 

 continually dividing to form new cells 

 on the outer side (bast) and on the inner 



side (wood) of the cambium-layer ; the wood contains but few vessels. 

 (6) Study and compare the roots of Carrot, Parsnip, Beet. The 



Beet root is peculiar in having several layers of cambium instead of 



Fig. 62. Tuberous Hoots of the 

 Dahlia. 



