Leaf structure of Cananga odorata (Annonaceae) in relation to the collection of photosynthate and phloem loading: morphology and anatomy

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Abstract
Four distinct anatomical types of minor veins occur in Cananga odorata leaves. In order of decreasing size, they are (i) type I, with tracheary elements, fibers, vascular parenchyma cells, companion cells, and mostly nacreous-walled sieve-tube members; (ii) type II, with the same cell types except that the sieve-tube members have walls that usually lack nacreous thickenings; (iii) type III, with only vascular parenchyma cells and tracheids; and (iv) type IV (vein endings), with tracheary elements only. The proportions of the total minor vein length occupied by each are type I, 15.1%; type II, 27.2%; type III, 24.4%; and type IV, 33.3%. Thus about 60% of the minor vein network lacks sieve tubes. The average interveinal distance for all minor veins is 121 μm, but the average for veins containing sieve-tubes is 329 μm. Other salient features include vascular parenchyma cells up to 130 μm long, bundle-sheath cells whose lateral protuberances into the mesophyll increase extensively with decreasing vein size, and five layers of horizontally oriented spongy parenchyma cells. These features may facilitate transport of assimilate to the relatively small proportion of the minor vein network that contains sieve tubes.