140 ROOTS 



spring and harvested in the same growing season. Shep- 

 herd's-purse and pepper-grass are common weeds which 

 show this same habit of being sometimes annual and some- 

 times biennial. Apparently, hardy plants which are 

 usually annuals are capable of Kving as biennials if their 

 seeds sprout in late summer. Similarly there are plants, 

 usually biennials, which behave as annuals if they get 

 their start very early in the spring ; that is, they will com- 

 plete their life history and die before winter comes. Also 

 some plants, like the castor bean, are annual in temperate 

 regions and perennial in tropical regions. 



Biennials usually devoie their energy in the first season 

 to the manufacture of food and its storage; they usually 

 give no attention to reproduction. Food is stored in the 

 underground parts. The next season, using this surplus 

 of food, the plant produces flowers, seeds, and fruit, and so 

 completes its Hfe history. The burdock is a common 

 biennial weed which has the habit just described. In try- 

 ing to get rid of weeds, it is evidently important to know 

 which weeds are annual and which are not. Only annual 

 weeds are destroyed by destroying their tops. Carrots, 

 beets, and parsnips are biennials. In the first season of 

 growth they develop a large bunch of leaves, but not much 

 stem. The food is stored in the tap-root. If this is left 

 in the ground, next season the growth of the stem will be 

 much greater. Flowers and fruit will be produced, and the 

 fleshy root will wither. Its food will have been used up. 



45. Contraction of Roots. — - Some plants have roots 

 which seem constantly to be pulling the stem down into 

 the soil. If you have ever dug dandehons, you have noticed 

 how tightly the center of the cluster or rosette of leaves 



