Abstract.The export of assimilates from mature leaves towards the young leaves was investigated: 100% and 65% of constitutive matter of leaves of rank 0 (the youngest leaves at the top of the plant) and of rank 1, respectively, originated from other parts of the plant. Photosynthesis of a particular leaf covers the total carbon requirement of that leaf only when it reaches about two‐thirds of its mature size. When pairs of mature leaves were excised, the young leaves increased their own autotrophic growth while the level of assimilates exported by the remaining leaves remained unchanged. The existence of permanent pools in the leaves that export the assimilates was demonstrated; about 50% of the carbon, both in the soluble and insoluble fractions of mature and senescent leaves (ranks 5 to 8 from the apex), was not renewed by turnover. It is shown that the13C‐enrichment of the components of the starch‐malate sequence in young leaves results, at least in part, from the incorporation of imported carbon chains. The significance of the δ13C diagnosis in CAM determination is discussed in relation to the origin of the constitutive carbon of the