514 HORTICULTURE, FORESTRY, FLORICULTURE 



The importance and necessity of a proper amount of heat and 

 light in greenhouses are well understood. It is very often the case, 

 however, that the smaller details in matters of this kind are over- 

 looked or neglected, and the plants in consequence suffer. Different 

 plants, as is well known, require different temperatures for their best 

 development. These differences, as is also well known, vary not only 

 with different varieties and forms of plants, but also with the differ- 

 ent stages in the growth of the same. The plant in its relation to heat 

 has been likened to a steam engine. When the tension of the steam 

 is slight, the machine is barely able to overcome the friction of its 

 own parts, and under such circumstances can do little or no work. 

 As the tension of the steam is increased, the efficiency of the engine 

 becomes greater and greater, until finally it reaches a noint where 

 the very best work is done. If the tension of the steam is increased 

 beyond this point, the parts of the machine become strained, and 

 the whole will eventually break down unless relieved of the pressure 

 put upon it. In the case of a plant there is a point in the temperature 

 barely sufficient to awaken the vital energies of the organism. With 

 increasing heat the vital forces of the plant increase, until a point is 

 reached when the best growth is made; beyond this point the plant 

 suffers, and is eventually killed if the temperature continues to 

 increase. 



In considering the question of heat, the importance of soil tem- 

 perature and its relation to the temperature of the air must not be 

 overlooked. Unless the proper conditions are maintained in this 

 respect, an ideal development can not be reached, and the plants, in 

 addition to developing characters that make them unprofitable, are 

 frequently made more subject to disease. A striking example of the 

 latter is found in the case of lettuce when forced under glass. At 

 certain stages of growth the plant in question is much subject to 

 burn or scald, and for this reason it is often rendered wholly unfit 

 for market. The burn is primarily brought about by the rapid 

 evaporation of moisture from the leaves at a time when the roots 

 are not able to supply the demand for water. The temperature of 

 the soil has a marked effect on root action, and in this way the sup- 

 ply of water made available to the leaves is influenced. If the soil 

 is cold, or, in other words, if the relation of its temperature to that 

 of the air is improper, the roots can not furnish the water as fast as 

 it is needed, and in consequence the tender tissues of the plant above 

 ground simply collapse. 



The value of light in the growth of plants is not always fully 

 appreciated. It is a common occurrence to see plants which require 

 strong light for their development struggling for existence in dark 

 houses half buried in the ground. Within recent years, however, 

 there has been a marked improvement in the manner of constructing 

 greenhouses, and there is no doubt that the improvement in many of 

 the crops now grown can be attributed to the recognition of the fact 

 that properly regulated light is one of the fundamental factors in 

 the growth of crops under glass. 



