34 



Commercial Gardening 



phosphorus, nitrates, &c.; and iron in a variety of compounds. Most of 

 the above are present in sufficient quantity in soils generally, and when 

 land requires manuring, nitrogen, phosphorus, and potash are usually most 

 deficient. Lime is occasionally deficient, and is useful for a variety of 

 purposes. Except in the case of Leguminous crops, such as Peas, Broad 

 Beans, Dwarf Beans, and Scarlet Runners, nitrogen is always necessary 

 unless the soil is very fertile. Leguminous plants have bacteria in small 

 nodules upon their roots, and these bacteria are capable of fixing the free 

 nitrogen of the air. Farmyard manures are very valuable in light soils 

 by increasing their power of holding water, independently of the plant 

 food they contain. 



Contractile Roots. Apart from the functions already described, a 

 large number of bulbous plants are provided with roots which have the 



Fig. 18. Seedling Plant of Gourd (Cucurbito Pepo) with Radicle, Caulicle, and opposite Cotyledons. 

 Liberation of the Cotyledons from the cavity of the Seed or Fruit Husk, showing in the central figures 

 the little peg or radicle that serves to fix the seedling. 



power of contracting at certain periods, and thus pull down the bulbs or 

 corms deeper into the soil. These roots are known as " contractile ". They 

 are generally thicker and fleshier than the more fibrous feeding roots, and 

 are recognized by the transverse wrinkles or rings upon them. The young 

 or new corms of Gladiolus and Crocus, and the young bulbs of many Liliums 

 and other bulbous plants, are all provided with such roots. In the case of 

 seedlings, Dr. Scott says, in his Structural Botany, that the young bulb 

 "is gradually drawn down year by year owing to the shortening of the 

 adventitious roots. As the end of the root attaches itself firmly to the 

 soil, the effect of the contraction is to exert a downward pull on the bulb. 

 The upper part of the root is alone capable of contraction, and is much 

 thicker than the rest. The inner cortex is the actively contractile tissue; 

 as it contracts, the external layers are thrown into transverse wrinkles. 



