THE BOTANY OF THE APPLE TREE. 13 



THE TREE IN HEALTH. 



The healthy apple tree derives a portion of its food from the soil 

 into which its roots penetrate. This food consists of various sub- 

 stances, the chief of which is water, in which all the other food sub- 

 stances are dissolved. No substance is taken up by the roots in a 

 solid state, but in every case the food from the soil must be dissolved 

 in water before it can be absorbed. The particular organs of absorp- 

 tion are the younger portions of the small roots, and the minute hairs 

 which clothe the surface of new roots. The statement still to be seen 

 in popular books, and often repeated in some journals, that the watery 

 matters in the soil are taken up by "spongioles" on the tips of the 

 roots is a gross error. There is no such thing as a "spongiole" at 

 the end of any of the roots, and it is strange how so palpable an error 

 should have been continued for so long. The very tip of the root 

 where the " spongiole " was supposed to be takes no active part in the 

 absorption of food matters from the soil. 



In addition to water itself, the other food-matters, are (1) some nitro- 

 genous substances, (2) salts of lime, (3) salts of potash, (4) salts of 

 phosphorus, (5) salts of iron, and probably small quantities of a few 

 others. The nitrogenous substance may be a salt of ammonia, such as 

 would be supplied by barn-yard manure, or it may be a nitrate such 

 as manure supplies, as well as many of the commercial fertilizers. 

 Most soils contain enough of the salts of lime and potash for the 

 supply of the tree, but they do not all contain enough of the phos- 

 phorus and iron, and these may profitably be added to the soils in 

 small quantities. Decaying bones and rusty scraps of iron in the soil 

 will supply all that the tre requires. 



The supply of food which the tree derives from the air is in certain 

 respects more important than that which it obtains from the soil. 

 The green leaves absorb carbon dioxide (commonly called carbonic 

 acid gas) from the air, in which it exists in minute quantities. Car- 

 bon dioxide is a heavy, poisonous gas, composed of carbon and oxygen. 

 It is produced in the burning of wood, oil, illuminating gas, and in fact 

 it is one of the products of the burning of nearly all common substances. 

 This gas is absorbed by the green leaves, entering through the little 

 pores found in great numbers on the under surface. It may not be 

 generally known that upon every leaf there are immense numbers of 

 mouth-like pores, which open and close according as the conditions 



