110 VIII. THE MONOCOTYLEDONOUS STEM. 



This is a sieve-plate covered with Callus, the structure of which 

 we shall study hereafter on other and more favourable objects. 

 The callus-plates absorb the corallin with special avidity, and 

 therefore stand out so sharply stained. By the side of the sieve- 

 tubes can be distinguished the companion-cells (s). They are 

 narrower and shorter than the sieve-tubes, and have besides 

 richly protoplasmic contents, and a readily visible nucleus, for 

 which we look in vain in the sieve-tubes. Sclerenchyma fibres 

 of the vascular bundle sheath, and with very oblique end-walls, 

 bound the bundle towards the exterior. The innermost cells of 

 the sheath have, as the cross-section has already shown, a com- 

 paratively broad cavity. Starch grains are not found in the cells 

 of the vascular bundle ; in this case, however, they are wanting 

 in the cells of the ground-tissue also. All the cells of the vascular 

 bundle and of the ground-tissue, with the exception of the tracheae, 

 trachei'des and sieve- tubes, contain nuclei. 



It is clear that such a median longitudinal section of the bundle 

 as is described above, can show neither of the two great vessels 

 the pitted ducts. If the section be not very thin, one may show 

 by deeper focussing, but it is indistinct. In order to study the 

 longitudinal section of one of the great vessels, we must look for 

 a section which cuts the vascular bundle out of the middle line. 

 We shall then see that the great vessel is obliquely pitted, less 

 often spirally or reticulately thickened. In these pitted ducts the 

 thickened parts form a network. The pits broaden out at their 

 bottom, but are, however, only unilaterally "bordered," in that 

 the corresponding pit of the adjoining cell of the wood parenchyma 

 is wanting in a "border". These cells, too, are less thickened 

 than the vessels. The diaphragms, or cross-walls, of the great 

 ducts at once attract attention in the longitudinal section. They 

 show a double ring, which only projects a slight distance into the 

 cavity of the duct. These rings originated in a thickening of the 

 outer edges of the cross-walls, the inner unthickened part of 

 which was afterwards dissolved (resorbed). From the number 

 of the diaphragms we can therefore draw a conclusion as to the 

 number and size of the cells of which the duct is composed. At 

 the place of insertion of the diaphragm, the vessel shows a slight 

 constriction. 



Double Staining. 1 It is very instructive to attempt double 



1 Stains used for cell-wall staining can be classified as those which stain 

 (1) lignified membranes only, (2) unlignified membranes only, and (3) mem- 



