1 82 Practical Plant Biology. 



incomplete partitions extend into the cavity of the cell, dividing 

 the cell up into a number of incomplete compartments. In 

 these cells are found large numbers of chloroplasts embedded 

 in the film of protoplasm lying against the wall. Between the 

 isodiametrical mesophyll cells and the epidermis there is a sheet 

 of elongated fibres which is continuous under the epidermis, 

 except below the stomata, where it is replaced by V-shaped 

 mesophyll cells enclosing between their two branches the cavity 

 which leads through the stoma to the outside. The mesophyll 

 is bounded on its inside by a uniform layer, one cell thick, 

 the endodermis. Its cells are oblong and fit closely together. 

 The tissue between the conducting tracts and the endodermis 

 is called the pericycle. It is composed of some cells with 

 cellulose walls filled with proteins and starch, and of others 

 whose walls have become woody or lignified and have lost their 

 protoplasmic contents and are, in fact, tracheids. The walls 

 of these tracheids are furnished with bordered pits, the nature 

 of which we will study later. These two kinds of cells form 

 the transfusion -tissue, which is analogous in its functions to the 

 finer ramifications of the vascular bundles of other leaves. The 

 two conducting tracts are composed of parallel strands of wood 

 and bast. The wood is placed next the flat side of the leaf 

 and the bast towards the convex side. The former is built u> 

 of a number of fine capillary-tubes with walls of lignified cellulose 

 (woody substance), and the latter of long fine cells with thin 

 cellulose walls. These latter, the sieve-tubes, have areas (sieve- 

 plates) on their walls perforated with very minute pores, and 

 through these the mucilaginous contents of the sieve-tubes are 

 continuous. The two conducting tracts run down the leaf 

 enclosed in the transfusion-tissue and pass without any marked 

 change in their direction into the dwarf shoot. Here the four 

 strands contributed by the two leaves remain separate for a short 

 distance, but gradually converging, they soon fuse together and 

 form a tube surrounding the narrow pith. This tube formed 

 of the downward prolongation of the conducting tracts of the 

 leaves enters the cortex of the stem which supports the dwarf 

 shoot obliquely. As it approaches the pith it divides into two 

 strap-like portions which bend downwards and run in a steep 

 spiral on the outside of the pith towards the base of the stem. 

 In their course downwards the two straps fuse with the bundles 

 coming from other dwarf shoots. Owing to these fusions, the 

 number of bundles in the stem at any level, except at the very 

 top, is less than the sum of the bundles contributed by the dwarf 



