114 ACRONOMY 



exclusion of most of the moisture. Some hardy arctic plants, 

 however, can be frozen and thawed several times a day during 

 the growing season without being injured. 



The plants of a given region have their own peculiarities 

 in the matter of the temperature at which growth processes 

 begin. In the arctics certain seaweeds thrive in water that 

 seldom rises above 32, and are easily killed by temperatures 

 a few degrees higher. Most plants of the temperate zone will 

 begin to grow at about 41 above zero, and some, such as 

 oats, wheat, rye, and peas, can make some growth when the 

 temperature is just above the freezing point; but the best tem- 

 perature for germination is between 60 and 70, and many 

 species, even in the colder parts of the world, will not start 

 to grow until such temperatures are reached. Up to a certain 

 point heat seems to stimulate growth processes just as it does 

 chemical reactions. In the tropics the temperature at which 

 seeds germinate is usually ten or twelve degrees higher than 

 that required for more northern plants, the most desirable being 

 between 70 and 80. The seeds of many tropical species, 

 when planted in our hothouses, must be given a temperature 

 above 90 to get the best results. These facts explain why 

 some seeds are planted earlier than others. Peas and spinach 

 are cool-weather plants and may be planted as soon as the 

 ground can be worked in spring; indeed, unless the season 

 is fairly cool these crops do not do well. Corn and tomatoes, 

 on the other hand, which came originally from the tropics, 

 must wait until both the soil and air are thoroughly warmed. 



Hardy and tender plants. As regards the sensitiveness of 

 the plants to cold, gardeners are accustomed to group them 

 as hardy, half-hardy, and tender species. Hardy plants are 

 those that endure the winter season unharmed. The perennial 

 plants of any region are necessarily hardy plants. Half-hardy 

 plants are those that need artificial protection during the 

 winter, though in mild seasons they may survive without this. 



