dee tee oS brictaval and Ph ysiological Botany. 
licglate or scalariform char acter (Fig. 69, s s). True vessels is here i 
‘become lignified, but are never branched, or at all events extremely 
rarely. In the mature state they contain nothing but air; but 
occasionally in the spring a portion of the sap sucked up by the roots is — 
pressed into these air-containing vessels, a process on which depends, 
for example, the ‘ weeping’ of wounded grape-vines. In many Vascular . 
Ciyptogams, Gymnosperms, and Monocotyledons, as wellas in a few 
Dicotyledons, rows of vascular cells are found in places where, from 
the analogy of other plants, one would expect to find vessels, the 
| partition-walls not having become absorbed. Such structures compose 
what is called a conducting téssue ; and the separate cells are not called 
vascular but conducting cells. 
Bast-tubes or bast-fibres are long pointed tubes—not 
cells, because their elements have completely coalesced— 
usually thick-walled, rarely branched. They are not often 
in direct communication with one another, and then always 
only by lateral branches. ‘They are commonly united into 
bundles, and as such form an essential part of many fibro- 
vascular bundles (see Chap. VI.). 
While in vessels the separate vascular cells of which they are com- 
posed can still be made out with tolerable ease, this is by no means the’ 
case in bast-tubes ; on which account opinion was long divided among 
botanists, whether bast-tubes—the development of which is generally 
extremely difficult to observe—are formed by the coalescence of cells, 
or by a single cell increasing from 20 to 50 times its original length. The 
simple consideration, however, that if the growth of bast-tubes were © 
of the latter nature, it must be accompanied by an increase in thickness 
of the entire bundle of which it formed a part, which is not actually the 
case, shows that they can arise only by the coalescence of cells. 
A close relationship subsists between  sieve-tubes, 
vesicular vessels, and laticiferous vessels, partly on account 
-of their form, partly of their contents ; all appearing to 
have for their function the storing up of nutrient fluids, 
and the conducting of them to the parts of the plant where ~ 
they are required. 
Sieve-tubes or bast-vesseds result from the coalescence of 
cells standing one over another, the partition walls of which, “ 
orszeve-discs, have become perforated in the manner of a ‘sieve 5. am 
