36 THE FLOWERING PLANT. 
steve 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 histulogy 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. Hach 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 
