6o THE VEGETATIVE ORGANS 



Celandine (Fig. 28) and Dahlia have swollen or tuberous 

 roots. Such swollen roots serve as important food-stores 

 for the plant, and some of them, if tested with iodine solu- 

 tion, will be found to contain much starch. Others, like 

 Beet, contain cane sugar, and the Dahlia contains an allied 

 substance called inulin. Frequently plants produce roots of 

 more than one kind and serving different functions, (1) some 

 being fibrous, absorbing roots ; (2) others swollen and stored 

 with food (e.g. Lesser Celandine and Dahlia). Some have 

 roots which after greatly elongating, contract and pull the 

 stem down deeper into the ground, e.g. Dandelion, Crocus 

 (Fig. 84), Bluebell (Fig. 87). The adventitious roots of the 

 Ivy, which arise in clusters on the aerial stems, serve rather 

 as holdfasts and climbing organs than for the purpose of 

 absorption. The roots of some water-plants like the Duck- 

 weed (Lemna) (Fig. 26, 5) and Frog-bit (Hydrocharis) dangle 

 in the water, from which they absorb nutriment, and do not 

 enter the soil. They are truly aquatic. Those of some 

 tropical aquatic plants contain large air-spaces and serve 

 as floats. 



Aerial roots. Roots, though rarely green, do sometimes 

 develop the green colour characteristic of leaves, as in the 

 roots of a few water-plants such as the Duckweed, and in 

 the aerial roots of Orchids. Many tropical Orchids, growing 

 perched on the trunks of trees, produce roots of three kinds : 



(1) holdfasts, which fix the plant like a bracket to the tree ; 



(2) long aerial roots which hang down in, and absorb mois- 

 ture from, the air ; and (3) nutritive roots, which grow 

 among, and absorb substances from, the humus that collects 

 on the bracket of leaves. 



Adventitious shoots : suckers. One of the most constant 

 characteristics of roots is that they give rise to members 

 similar to themselves, viz. root-branches. Thus they differ 

 'from stems, which produce members unlike themselves, 

 viz. leaves, which are usually green. It often happens, 



