32 Commercial Gardening 



liquid form. If some of the latter is poured on a clay soil, it can abstract 

 ammonia, free potash, phosphoric acid, and various salts containing plant 

 food and hold them till they are absorbed by plants. All clay soils, if 

 not originally fertile, can readily be made so by artificial means, and it 

 only remains for the cultivator to make them sufficiently porous by good 

 tilth to enable the roots of cultivated plants to penetrate freely and collect 

 the food stored. 



Water and Air Roots. While the roots of land plants can only absorb 

 the film of water adhering to the particles of soil, the roots of water plants 

 are able to absorb the free water with which they are surrounded. They 

 are greatly elongated, much more branched than those of a land plant, 

 and thin -walled, without cuticle or root hairs on their surface. A land 

 plant may produce water roots, as when Hyacinths are grown in glasses 



of water. Another good instance in nature may 

 often be seen where the roots of trees penetrate 

 tile drains and actually choke them up. If the 

 roots of a land plant are immersed in a vessel of 

 water they continue to absorb water for a time, but 

 soon develop true water roots and the earlier or 

 original fibres die. They get their food and air 

 dissolved in the water surrounding them. One 

 peculiar form of air root may be seen in Orchids. 

 The root is surrounded by a membrane of cells, 

 several layers deep, more or less thickened, per- 

 forated with holes, and filled with air. They ab- 



17. -Dahlia-Tuberous Root sorb rain containing plant food in solution, and 

 deposited at first in the form of dust on or near 



the root. If such roots develop on the outside of a flower pot or basket 

 they must not afterwards be buried either in soil or Sphagnum. The 

 writer has seen a fine batch of Moth Orchids (Phaleenopsis) killed by 

 placing the small baskets inside larger ones and filling the space between 

 with Sphagnum. In many Aroids that produce aerial roots the surface 

 is loose and spongy and more or less densely covered with root hairs 

 which absorb moisture from the air. 



Tuberous Roots. The primary and secondary roots of the Dahlia 

 become greatly swollen and spindle-shaped (fig. 17). The thickened 

 portion is intended for the storage of reserve material with which to 

 make a good start the following season in the production of the flower 

 stem. The material stored is inulin. The base of the stem and the upper 

 part of the root of the Turnip becomes greatly thickened and tuber-like, 

 storing starch for the requirements of the flower stem in the second 

 season. In the case of the fleshy, thickened taproots already mentioned, 

 the Carrot and Parsnip store starch for the same purpose as the Turnip, 

 and, all being good for food, they are cultivated for this special purpose 

 by man. The same applies to Beet, which stores a sugar very like cane 

 sugar. 



