116 VIII. THE MONOCOTYLEDONOUS STEM. 



a sheath is not clearly defined ; yet it can be seen that the cells 

 of the ground-tissue nearest to the vascular bundle are smaller, 

 and that they join together without a break. 



By a scrutiny of the tissue in the neighbourhood of the vas- 

 cular bundle it will be seen that single small cells, between the 

 large cells of the ground- tissue, contain a highly refractive crystal 

 (Fig. 42, k). It offers itself to us here in cross-section, or end 

 view ; as to its longitudinal form we can readily inform ourselves 

 by suitable sections. Similar crystals, contained in narrow cells, 

 can likewise be seen here and there interposed amongst the larger 

 cells of the general tissue of the leaf. 



A longitudinal section through the leaf, cutting through the 

 middle of a bundle, shows us at the inner limits of this bundle 

 the greatly stretched, partly crushed, spiral vessels (or vascular 

 iracheides) which we saw in cross- section at ss, and dis- 

 tinguished as protoxylem elements, i.e., as the first developed 

 elements of the wood portion of the bundle. Following are 

 broader more closely wound spiral vessels (or trache'ides), and 

 then narrow elements with scalariform thickening. In the bast the 

 sieve-tubes show clearly only when the sieve-plates have been 

 stained with corallin. Farther outwards the sclerenchyma fibres 

 are recognised by their strong thickening, their notable length, 

 and pointed ends. 



In order to control the results hitherto obtained, take some 

 cross-sections from a living leaf. We then note that the large 

 cells of the ground-tissue in the- outer parts of the leaf contain 

 chlorophyll grains, while the cells sheathing the vascular bundle 

 are devoid of chlorophyll. In fresh preparations the vessels are 

 filled with air, whence the structures are less clear than in 

 alcohol material. 



Crystals in Iris. As the crystals are directed parallel to the 

 long axis of the leaf, they show in profile in longitudinal sections 

 (Fig. 43, A, D). They lie in elongated cells of the ground- tissue, 

 which are only a little larger than the crystals themselves. 

 These cells contain no chlorophyll, while the neighbouring cells 

 usually contain chlorophyll. The crystals in question dissolve 

 readily if treated with hydrochloric acid, without evolution of gas ; 

 whence we readily conclude that they are composed of oxalate of 

 lime. All the crystals occurring here have an elongated prismatic 

 form, and belong to the monoclinic system ; most of them appear 

 geminate (twin crystals), (D). 



