The secondary walls of xylem elements are resistant to these enzymes and these cells simply break when the others have separated. This is particularly useful for mechanically harvested crops like coffee or for thinning excess crop on fruit trees. This cabbage was stored in the same room as apples, which produce large amounts of ethylene. The ethylene stimulated abscission of the leaves from the stalk, even though they were immature. In perennial plants senescence of some of the plant structure is associated with dormancy in the over-wintering structure.
Dormancy is a phase in which cell division and cell expansion are suspended.
In these situations growth can be restored by correcting the environmental problem. In true dormancy plants will not grow even if they are given optimum environmental conditions. Dormancy sets in at the end of the growing season as the days get shorter and temeperatures fall. To emerge from dormancy plants generally require a period of low temperature, weeks in the case of herbaceous perennials to months for many trees. After this cold requirement has been met the plant will grow again when environmental conditions become favorable.