DEMANDS ON A PLANT TRANSPORT SYSTEM

Delivery to all tissues …

Growth can take place in any area of a plant throughout its lifetime (see Part III). However, only green parts of the plant can photosynthesize yet non-green parts also require food to provide energy and other needs. For example, buds at the tips of branches contain many young leaves which are too immature to carry out photosynthesis yet are growing rapidly. The trunks and stems of trees and many shrubs are not green yet contain substantial amounts of living tissues which must be provided with nourishment. Energy is needed to form flowers, fruits, and seeds, none of which may be green, at least when mature. Some plants form food storage organs like bulbs, corms, and tubers, to which, as they swell, food must be delivered.

… At different times

These various demands for food arise not only at widely separated locations in a plant but also at different times during a growing season. In temperate climates early in spring the main need may be to move food from mature leaves capable of photosynthesis or from storage organs to young, developing leaves at stem tips, and to rapidly growing roots. Later, as the plant begins reproductive growth, flowers must be nurtured, followed by fruits and seeds. Later still, if overwintering storage organs are formed they, too, must be supplied with significant amounts of food. All of these organs can be at different locations on the plant body.