166 AMERICAN POMOLOGY. 



plants. The loss of out- fruit crops in some parts of the 

 Ohio Valley in the years 1363, 1865, and 1866, was fairly 

 attributed to this cause. 



We must not overlook the unhealthy influences produced 

 by an excess of moisture in the earth. Many plants that 

 naturally delight in a dry porous soil, become weak, un- 

 fruitftil, or even seriously diseased when they are planted 

 in low wet grounds, or upon such as are underlaid by a 

 very tenacious sub-soil, while an opposite condition is 

 equally unfavorably to those that .ire naturally more 

 aquatic in their tastes and habits. In the former case we 

 learn to avoid such soils and situations, unless we are 

 able to change their character in this respect by thorough 

 under-dr.iining, which will completely remove the evil, and 

 the remedy becomes merely a question of expense. 



A certain amount of temperature may be assumed as 

 requisite to every plant, or rather it may be affirmed that 

 some plants cannot exist and thriva except within a certain 

 range, and it has been asserted that each class of plants 

 requires a mean temperature for the year that shall not 

 vary many degrees : the range of tliis variation has per- 

 haps never been satisfactorily ascertained. But it is well 

 known, tliat both heat and frost act injuriously upon vege- 

 tation. Mr. Lindley tells us that " the extreme limits of 

 temperature which vegetables are capable of bearing, 

 without destruction of their vitality, have not been deter- 

 Inined with precision." When the temperature is main- 

 tained at a higher point th.an is natural, the plant is ex- 

 cited to undue activity of growth ; but this is attended 

 with an enfeebled condition, often seen in badly managed 

 green-houses. Mr. Knight found that certain plants wei« 



