Necessity of Transport in Phloem
Leaves produce photoassimilates and support various tissues including roots. The excess photoassimilates and metabolites stored in specialised tissues produce fruits and seeds which germinate and regenerate the plant in the appropriate season. The leaves at the canopy of a tree are at a distance from roots. This can vary from a few millimetres to more than 100 meters. All in all we need an extensive transport system to carry the products of photosynthesis and nitrogen metabolism over medium and long distances. Diffusion is too slow a process to achieve this task over such large distances; what is required is a convective flow in the specialised vascular system so that the photosynthates reach all the needy tissues. The transport system has to be as extensive and ramified as the arterial and venous network in an animal body.
However, plants are devoid of a specialised pump such as heart meant for blood circulation. Though we have not yet fully understood how plants achieve this goal, they do carry out this exceedingly difficult task without a heart. Leaves manage to feed the tissues over vast expanses of the plant body with photosynthates necessary to fulfil their requirement.