128 THE VEGETATIVE ORGANS 



canals and railways. 1 In these ways the underground 

 parts of plants become valuable sand- and soil-binders and 

 play an important part as ' land-winners '. 



Thickened rhizomes. Examine the rhizome of the Solo- 

 mon's Seal or the Iris. The axis is greatly thickened and 

 bears many branched adventitious roots ; the internodes 

 are very short, and the scale-leaves and buds are large. 

 Cut a slice across the rhizome and examine the tissues. 

 Outside is a layer of cork, then a thick cortex, and 

 near the centre a number of scattered vascular bundles. 

 Place on the cut surface a drop of iodine solution and note 

 the large amount of starch stored in the cells. As the plant 

 grows, the rhizome tends to rise to the surface of the 

 ground, and if you examine Irises in a garden you will 

 often find that the soil is washed away from the rhizomes. 

 On plants which have thus approached the surface, it is 

 common to find thick unbranched roots which penetrate 

 the soil deeply, then contract and pull the rhizome down- 

 wards. Sometimes the growing end is directed downwards, 

 and as the rhizome elongates it descends until a suitable 

 depth is reached. Fig. 82 shows such a descending rhizome 

 of the Flowering Rush as it ploughs its way through the 

 mud in which it grows. 



Stem-tubers. The Potato is another strangely modified 

 hibernating organ. On the surface, which is covered with 

 a layer of brown cork, are small depressions, the ' eyes '. 

 Plant a potato, or even a thick slice of potato, in a pot 

 of soil, and as it grows you will find that shoots spring from 

 the ' eyes ', a fact suggesting that they are buds. Fig. 83 

 shows a plant grown in this manner. From the ' eyes ' (e) 

 leafy shoots have grown and the base of the stem has 

 produced numerous branched, fibrous roots. The buds 



1 On railway banks these plants often extend their bounds, 

 grow between the rails, and produce a weedy track very difficult 

 to keep clean. 



