36 THE FLOWERING PLANT. 



sieve tubes are different in structure from those of sunflower, and 

 much more difficult to study. On the other hand, there are dico- 

 tyledonous stems which are superior to sunflower for the study of 

 sieve tubes. In the cucumber and vegetable marrow, for example, 

 they are extremely large, and easy to see. These stems are, 

 however, abnormal in some respects, since there is not only bast 

 external to the wood, but also some internal to it. A band of 

 sclerenchyma will also be found in the cortex not far from the 

 epidermis. 



We now come to the histology of herbaceous monocotyledons. 

 In the stem of asparagus (fig. 7, D, E) we shall find epidermis of 

 the usual kind, while the ground-tissue is largely composed of 

 parenchyma. Each vascular bundle is made up of : (1.) Wood, 

 which in cross-section forms a V-shaped or U-shaped mass, with 

 apex pointing inwards. It is chiefly composed of vessels. As in 

 the sunflower, there is protoxylem, composed of spiral and annu- 

 lar vessels, which occupy the apex of the V. (2.) Bast, which is 

 almost all soft, and is situated on the outer side of the bundle, 

 chiefly within the limbs of the V. There is no cambium, and the 

 bundle is therefore closed, since no addition of new elements can 

 take place. Such closed bundles, with wood arranged in a V or 

 U, characterize monocotyledons generally. Examination of any 

 good- sized grass stem will show these features more clearly than 

 in asparagus. In addition, the whole bundle is surrounded by a 

 sheath of sclerenchyma, and there are also masses of this tissue 

 beneath the epidermis. 



Thickening of Stems. — It is the presence of cambium in the 

 stems of gymnosperms and dicotyledons that enables perennials 

 belonging to these groups to increase in thickness, sometimes to 

 a very large extent. This process may commence in herbaceous 

 stems, but has no time to go any great length. The increase is 

 mainly in the wood or xylem, and what are popularly called woody 

 stems owe their nature to this. But it must not be forgotten 

 that all stems contain a certain amount of wood or xylem, so that 

 the term herbaceous is not exactly opposite in meaning to the 

 term woody. If we take such a tree as a Scotch fir or elm, suc- 

 cessive examination of older and older stems will enable us to 

 understand how secondary increase of thickness takes place. We 

 will at first mainly consider the vascular bundles. A cross- 

 section through the axis of a bud will present a ring of primary 

 bundles, completely isolated from one another. This condition 

 is retained through life in some herbaceous stems, such as those 

 of the buttercup. The bundles are separated by strips of ground- 

 tissue, the primary medullary rays, and each of them consists of 

 primary bast on the outside and primary wood on the inside, the 



