THE ORGANIZATION OF THE PLANT 89 



more or less food in summer, which is expended in growth 

 during the following spring. The rhubarb and asparagus, 

 among garden vegetables, and the lily and iris, among decora- 

 tive plants, are good examples of this. Most of the asparagus 

 crop is produced from food made by the plant the preceding 

 year. There are two classes of perennials. In the herbaceous 

 perennials the stems die down to the ground in winter. Lilies, 

 peonies, asparagus, and rhubarb are examples of this class. 

 The woody perennials comprise our trees, shrubs, vines, and 

 other forms that put up stems, which continue to live for a 

 succession of years. Trees have a single trunk and usually 

 attain heights of more than twenty feet. Shrubs have several 

 stems and are less than twenty feet high. Bushes resemble 

 shrubs, though they are somewhat smaller, usually being no 

 taller than a man. Vines have stems too weak to stand alone 

 and consequently must be supported by stronger plants. They 

 may be root climbers like the poison ivy, twiners like the bitter- 

 sweet, true climbers like the grape and woodbine, or scramblers 

 like the climbing rose. 



The rest period of plants. In perennial species, after a 

 season of growth, the vegetative processes gradually cease, 

 buds are formed, the leaves are thrown off, the wood cells 

 thicken, and the protoplasm, excluding much of its moisture, 

 goes into a resting condition. It is likely that this season of 

 dormancy was originally in response to a change in the season, 

 for in northern regions it occurs at the beginning of winter 

 and in the tropics at the beginning of the dry season. A 

 period of rest, however, seems natural to most plants. Many 

 seeds refuse to grow if planted as soon as ripe ; the spring 

 flowering plants will not respond to warmth when brought 

 into the house in early winter ; and potatoes, onions, parsnips, 

 and the like, stored in cellars, do not begin to sprout until 

 the approach of spring. The hyacinth, narcissus, crocus, and 

 tulip, after flowering in the open ground, ripen their foliage 



