3<D THE FLOWERING PLANT. 



with blunt needles, it will be seen that the vascular bundles are 

 firm strands which traverse the ground-tissue in the long direc- 

 tion. With a little care it can be shown that at the node the 

 bundles are more or less united with one another, while fresh 

 bundles run iD from the leaf. All the bundles are common to 

 stem and leaf, since any one of them commences in a leaf and 

 then runs downwards in the outer part of the stem (cf. fig. 5, D). 

 The above description is applicable to most herbaceous gymno- 

 sperms and dicotyledons. Similar observations may with advan- 

 tage be made on the broad bean ( Vicia faba), where, however, the 

 stem is square and glabrous, the bundles follow the external shape, 

 and the pith is traversed from end to end by a large air cavity 

 (cf p. 23). That this has been caused by rupture of the internal 

 tissue may be seen by the presence of ragged fragments on the 

 walls of the cavity. A very young green stem of the Scotch fir 

 may also be examined, but the parts are much smaller and more 

 difficult to see. 



The asparagus stem is a typical monocotyledonous one, and a 

 cross- section shows that the vascular bundles are not arranged 

 in a ring, but scattered through the ground-tissue, so as to forbid 

 the existence of sharply defined cortex, pith, and medullary rays 

 (fig. 7, D). As in the previous case, they are common, but instead 

 of running directly downwards, run inwards and downwards, 

 thickening as they go, and then take an outward and downward 

 course (cf. fig. 5, C). The bundles are connected together at the 

 nodes by numerous cross branches. Owing to the way in which 

 the bundles thicken, it is clear that those nearest the centre at 

 any point should appear largest in a cross-section taken at that 

 point, and this is actually seen to be the case. The course of the 

 bundles cannot be followed with the same ease as in the sunflower. 

 The hollow stems of grasses bear a similar relation to the aspar- 

 agus stem that the bean stem does to the sunflower stem. The 

 nodes, however, are solid, as can easily be shown by splitting the 

 stem longitudinally. 



We now come to the histology of the two herbaceous stems 

 taken as examples, and it is necessary to explain that longi- 

 tudinal sections are of two kinds, radial, corresponding with a 

 radius of the cross-section, and tangential, at right angles to this. 

 The cells of the epidermis in the sunflower stem are flattened, 

 and their cellulose walls thick externally. A very thin mem- 

 brane, the cuticle, covers the outside of the epidermis, but though 

 part of this, it is not divided into areas corresponding with 

 the cells, and may perhaps be regarded as a hardened surface 

 excretion. The substance cutin, of which it is composed, contains 

 the same elements (C, 0, H) as cellulose, but in different propor- 



