DICOTYLEDONS. 177 



ring, and the irregular arrangement of the tissues derived from its growth, 

 soon obliterate all traces of the primitive arrangement of the tissues. 

 Making a thin cross-section of the stem, and magnifying strongly, we find 

 bounding the section a single row of epidermal cells (Fig. 94, A, ep.) 

 whose walls, especially the outer ones, are strongly thickened. Within 

 these are several rows of thin- walled ground- tissue cells containing numer- 

 ous small, round chloroplasts. The innermost row of these cells (sh.) 

 are larger and have but little chlorophyll. This row of cells forms a sheath 

 around the ring of fibro- vascular bundles very much as is the case in the 

 horse-tail. The separate bundles are nearly triangular in outline, the 

 point turned inward, and are connected with each other by masses of 

 fibrous tissue (/), whose thickened walls have a peculiar, silvery lustre. 

 Just inside of the bundle sheath there is a row of similar fibres marking 

 the outer limit of the phloem (ph.). The rest of the phloem is composed 

 of very small cells. The xylem is composed of fibrous cells with yellow- 

 ish walls and numerous large vessels (tr.). The central ground tissue 

 (pith) has large, thin- walled cells with numerous intercellular spaces, as 

 in the stem of Erythronium. Some of these cells contain a few scattered 

 chloroplasts in the very thin, protoplasmic layer lining their walls, but 

 the cells are almost completely filled with colorless cell sap. 



A longitudinal section shows that the epidermal cells are much elon- 

 gated, the cells of the ground tissue less so, and in both the partition walls 

 are straight. In the fibrous cells, both of the fibro- vascular bundle and 

 those lying between, the end walls are strongly oblique. The tracheary 

 tissue of the xylem is made up of small, spirally-marked vessels, and 

 larger ones with thickened rings or with pits in the walls. The small, 

 spirally- marked vessels are nearest the centre, and are the first to be 

 formed in the young bundle. 



The epidermis of the leaves is composed of irregular cells with wavy 

 outlines like those of the ferns. Breathing pores, of the same type as 

 those in the ferns and monocotyledons, are found on both surfaces, but 

 more abundant and more perfectly developed on the lower surface of the 

 leaf. Owing to their small size they are not specially favorable for study. 

 The epidermis is sparingly covered with unicellular hairs, some of which 

 are curiously branched, being irregularly star-shaped. The walls of these 

 cells are very thick, and have little protuberances upon the outer surface 

 (Fig. 93, E). 



Cross-sections of the leaf may be made between pith as already directed ; 

 or, by folding the leaf carefully several times, the whole can be easily 

 sectioned. The structure is essentially as in the adder-tongue, but the 

 epidermal cells appear more irregular, and the fibro- vascular bundles are 



