DEVELOPMENT OP THE OVULES. 139 



contributing along witt the green walls of the pericarp 

 or seed vessel to the nourishment of the young embryo. 

 The vitality of all the organs of the flower is, however, ex- 

 hausted in succession before . the seed arrives at maturity 

 and the embryo is fully developed; the calyx and pericarp 

 alike perish, when they have fulfilled this the last and most 

 important function of vegetable life. An attentive observer 

 may watch these changes throughout the summer months, 

 in any plant that produces flowers and fruit, and may thus 

 satisfy himself of the general correctness of these state- 

 ments. 



At the period of flowering, a great exhaustion of the nu- 

 tritive juices of plants invariably takes place. In annual 

 herbaceous plants, the formation of the seed not only ex- 

 hausts the vitality of the flower, but of the stem and leaves, 

 and the whole plant perishes the first year. In biennials, 

 a store of nutritive matter is assimilated during the first 

 year, which is deposited in the root, and consumed by the 

 act of flowering the second year ; the plant therefore neces- 

 sarily perishes. In perennial herbaceous plants, which 

 invariably rise from an underground stem, the aerial portion 

 of the plant always dies down to the ground at the close 

 of the flowering season ; but the subterranean part remains 

 alive through winter, and as invariably re-appears above the 

 ground in spring ; therefore, only the upper part of such 

 plants is exhausted in flowering. But in woody perennials, 

 the formation of the flowers and fruit consumes only the 

 nutriment contained in the peduncle and its immediate 

 supports ; but the rest of the plant is not injured. The 

 first and succeeding year's growth of stem is therefore per- 

 sistent, or remains alive above the ground. It is true that 

 the tree sheds its summer leaves, and spreads its naked 

 branches to the wintry sky j but the stem, the numerous 



