TEMPERATURE, LIGHT, AND MOISTURE 115 



Tender plants are those that die as soon as frost comes. 

 These are general terms, however, and indicate relative con- 

 ditions only, since a plant that is perfectly hardy in one 

 region may be only half hardy or even tender in a colder one. 

 On the other hand, the trees that are deciduous in cold regions 

 may become evergreen when removed to warm regions. Vio- 

 lets, which flower for only a few weeks in spring in the 

 Northern states, may bloom throughout the autumn, winter, 

 and spring near the Gulf. 



Cardinal points. As has been indicated, there are three 

 important temperature points for every species of plant : the 

 minimum, or lowest point at which growth processes can pro- 

 ceed ; the maximum, or highest point at which growth is possi- 

 ble ; and the optimum, or most favorable temperature. These 

 points are called cardinal points, or the upper, middle, and lower 

 zeros. They are not the same for all plants, and in general are 

 higher for tropical plants than for those of temperate regions. 

 Each species may also have a different maximum, minimum, 

 and optimum for its vegetative and reproductive processes. In 

 such cases the cardinal points for growth are usually higher 

 than those for reproduction. The large number of species that 

 flower in early spring, often before the leaves have appeared, 

 are instances of this fact. 



Acclimatization. Some plants of tropical regions can be 

 induced to grow much farther north than they occur in nature, 

 and the same is true with respect to northern plants in more 

 southern regions. The adaptation of plants to such conditions 

 is called acclimatization. Complete acclimatization is possible 

 only with plants that are able to make new adjustments of 

 their cardinal points, raising or lowering them to fit the new 

 conditions. Sometimes the vegetative point may be thus 

 changed, but not that for reproduction, in which case the 

 plant may produce plenty of stems and leaves, but no flowers 

 or fruits. Our most persistent and successful weeds are largely 



