892 ECOLOGY 



plant from which the cutting is taken. At any rate, the maturity 

 of the flowering plant seems to be in some way transmitted to the 

 propagule. 



It has been proved conclusively that plants may be kept in a vegetative 

 state indefinitely, and that the usual successive stages in a plant's life 

 history are reversible. For example, when the ground ivy {Nepeta 

 hederacea), which commonly has a reproductive period intercalated 

 between the vegetative periods of spring and autumn, is grown in a green- 

 house under uniform conditions of moderate temperature and consid- 

 erable moisture, vegetative activity continues uninterruptedly. How- 

 ever, flowering may be induced at any time by transferring the plants to 

 a dry, well-lighted situation. Similarly, by exposure to proper external 

 conditions, " winter buds " have been induced in Vtrkularia at any 

 season, hyacinths have been induced to flower twice without an inter- 

 vening period of rest, and Parietaria has been kept in constant bloom. 

 Annuals have been transformed into biennials or perennials by keeping 

 them under constant conditions favorable to vegetative activity, and 

 Echium, which usually is a biennial, has been known to grow in the 

 tropics ten years v/ithout flowering. 



When the annuals, Poa annua and Senecio vulgaris, are transferred to alpine 

 habitats, the season is too short for fruit maturation and the plants become biennial. 

 Many garden annuals may be transformed into biennials by removing the flower 

 buds as they appear ; in this manner the mignonette may be transformed even into 

 a woody perennial. In monocarpic species life may be shortened by hastening 

 reproduction, as well as lengthened by promoting vegetative activity ; thus the castor 

 bean, a tropical perennial, has been transformed into an annual in temperate cli- 

 mates, where the conditions facilitate early reproduction. The closeness with which 

 death follows reproduction in monocarpic species is well illustrated in hemp, a dioe- 

 cious annual; the staminate plants die immediately after anthesis, while the pistil- 

 late plants live until the fruit has matured, several weeks later. A number of plants 

 which display vigorous vegetative reproduction (as the yam, the potato, and the 

 sweet potato) rarely produce seeds, hence it has been suggested that seed production 

 and vegetative production may be more or less mutually exclusive; however, there 

 are many plants in which both kinds of reproduction are vigorous (as in the dahlia, 

 the strawberry, and the willow). 



The usual succession of events from the inception of vegetative ac- 

 tivity to the maturation of fruit is so familiar that it has often been mis- 

 takenly referred to as normal, thereby implying that any change in the 

 order of events is abnormal. It has been shown that the order is re- 

 versible at almost any point. In certain species of Veronica, for example, 



