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5. pecial Morphology and Classification. 341 
essentially different from this, passing through the separate 
internodes in a parallel direction, while they bend irregularly 
and branch in the nodes. This occurs, for example, in 
the culms of Grasses, and may be compared with their 
course in Equisetacez (see p. 320). The pith also disappears 
at a very early period in these cases, from rupture and desic- 
cation, and probably also partly from resorption, so that 
the stem subsequently becomes hollow. 
The separate vascular bundles are in general more highly 
developed than in the classes already described ; but their 
increase in thickness 1s limited, and soon ceases. The xylem- 
portion of the bundles may consist of prosenchyma, vessels, ° 
and parenchymatous cells; the bast-portion of bast-fibres, 
sieve-tubes, and parenchymatous cells, with which are some- 
times associated laticiferous vessels. The prosenchymatous 
cells are sometimes more, sometimes less strongly thickened, 
but always lignified, and have a smaller number of scattered, 
slightly bordered, oval pits; sometimes they are pointed 
at the ends, forming a prosenchymatous tissue, and re- 
semble elongated parenchymatous cells. ‘The bast-fibres are 
of similar structure, but, as a rule, more pointed. ‘The 
vessels of the xylem-portion are annular, spiral, reticulated, 
or pitted. It is not uncommon also to find single vascular 
cells not united with others into true vessels. The lateral 
walls of the sieve-tubes are usually horizontal, and provided 
with sieve-discs ; the partition-walls, on the contrary, are 
mostly smooth, and only rarely, when united in. growth with 
similar ones, also penetrated by sieve-pores, thus forming 
conducting sieve-cells. The parenchymatous cells, both of 
the xylem- and of the bast-portion, are usually elongated, and 
hence resemble prosenchymatous cells ; but always have hori- 
zontal partition-walls, and are either not at all lignified, or, at 
all events, less so than the prosenchymatous cells. The 
laticiferous vessels are also sometimes found outside the 
vascular bundle ; and are then, when they contain raphides 
and belong to the cortex, called vesicular vessels, ‘The ar- 
