126 THE LEAF 



certain plants converge at their lower ends and rest upon the 

 upper dilated end of a cell of the spongy parenchyma. Ilaber- 

 landt regards these specialized cells of the spongy parenchyma as 

 collecting cells which receive the products of photosynthesis from 

 the palisade cells and transmit them directly or indirectly to the 

 main vascular channels. 



(c) The vascular system. Investigate first of all the general 

 structure of the midrib, and note particularly the absence of 

 photosynthetic parenchyma from this portion of the blade. In- 

 stead of the usual spongy and palisade parenchyma, the median 

 vascular bundle is covered on both sides by a number of layers 

 of isodiametric thick-walled cells, the outermost of which are 

 thickened in such a manner as to resemble coUenchyma. (Note: 

 This ''replacement" of photosynthetic parenchyma by col- 

 lenchynia and thick-walled parenchyma occurs to a less extent in 

 connection with the smaller vascular bundles of the Made. In 

 other types of leaves sclerenchyma may be present — mechanically 

 a ' ' girder effect ' ' is produced. ) Several smaller vascular bundles 

 with the phloem towards the adaxial surface are usually seen in 

 the upper part of the midrib; these ''accessory hioidles" unite 

 basally with the single leaf trace. A certain amount of secondary 

 growth has taken place in the median vascular bundle, a pheno- 

 menon which is of rather general occurrence in the larger veins 

 of dicotyledonous leaves. The conspicuous secondary xylem is 

 composed of conducting elements (probably both tradieids and 

 vessels) and fibers arranged in more or less definite radial rows 

 and uniseriate xylem rays which extend across the "cambial 

 zone" into the phloem. Tlie primary xylem (which lies above tlie 

 secondary xylem) consists of cells which show more or less of a 

 radial arrangement and which are imbedded among xylem paran- 

 chyma. The secondary phloem consists of polygonal thin-walled 

 sieve-tubes which are associated with small somewhat triangular 

 densely protojilasmic companion cells, phloem parenchyma, and 

 uniseriate phloem rays, each of which frequently terminates in a 

 large parenchyma cell. The primary phloem is indefinite and dif- 

 ficult to distinguish. The "cambial zone" can be distinguished but 

 is not nearly as i)i-oinineut or distinct as in llie case of a stem. 

 Nevertheless, three or more i-adial rows of differentiating cells 



