THE LIFE AND GROWTH OF SEEDLINGS 31 



pulled down by contractile roots, should consult Professor Oliver's 

 paper, " The Depth in the Soil at which Plants occur/' in the April 

 number of the Royal Horticultural Society Journal, 1898. 



AERIAL AND PARASITIC ROOTS. There are not many plants 

 with aerial roots in this country, but in the tropics they are very 

 common. Ivy climbs by means of the roots borne by its stem. 

 Like all roots, they are very sensitive to light, and like to get away 

 from it. The overshadowing leaf no doubt protects them from 

 too strong light, and it is in all probability the influence of light 

 which makes the roots penetrate every chink and cranny they 

 can, often to the great destruction of the wall. Many orchids have 

 aerial roots. In the tropics, orchids often grow on other plants 

 and climb from one to the other by their roots. This does not 

 injure the plant, for the food of the orchid is chiefly derived from 

 the dust which falls on the root, moisture being obtained from 

 the heavy dew. The orchid is therefore hardly a true parasitic 

 plant. In this country, there are many semi-parasitic plants, getting 

 their food partly from the air through the activity of their green 

 leaves, and also from the plant on which they are living. These 

 all send processes from their roots into the plant on which they 

 are living, and thus get part of their food from them. The red 

 and yellow rattle and the eyebright are semi-parasitic plants. 



The mistletoe is a true parasite, very fond of the black poplar 

 and apple trees. The seeds are dispersed by birds, chiefly thrushes, 

 which eat the mistletoe berries. The seeds become stuck on the 

 bark of the trees, and remain there some months before they 

 germinate. When germination begins, the radicle becomes 

 attached to the bark and puts out a process called a " sinker/' 

 which grows into the tree. 



The young seedling of the mistletoe is then fed by the food 

 taken from the tree. If it is growing on the poplar, its growth 

 is generally luxuriant, but if its " host " is a tree from which only 

 a limited amount of nourishment is derived, then the growth is 

 scanty. 





SUGGESTIONS FOR PRACTICAL WORK. The contents of a 

 greengrocer's shop will afford material for three or four lessons 





