52 PITTED VESSELS3—TYLOSES.—SPIRAL VESSELS. 
of a similar pitted nature is clearly shown in many instances by 
the contractions which their sides exhibit at various intervals, 
by which they acquire a beaded appearance (fig. 102) ; for these 
contractions evidently correspond to the 
points where the component cells come in 
contact, and in some cases even we find 
the intervening membrane not completely 
absorbed between the cavities, but re- 
maining in the form of a network or sieve- 
like partition (fig. 103). Pitted vessels 
generally terminate obliquely (fig. 103), 
and when they combine with neighbouring 
vessels, the oblique extremities of the 
latter are so placed as accurately to cor- 
respond with the former. In some cases, 
however, where the pitted vessels are 
pointed at the ends, they overlap more or 
less by these points. Pitted vessels may 
be commonly found in the wood of Dico- 
Fig.102. Beaded pitted ves- tyledons ; they are mixed here with the 
sel. Fig. 108. Pitted : 
vessel terminating ob. Ordinary wood-cells, but are much larger 
ee and Pre than these, as may be seen by making a 
as been dace cis transverse section of the wood of the 
absorbed. Oak, Chestnut, and other trees, when the 
holes then visible to the naked eye are 
caused by their section (fig. 183, v, v, v). The pitted vessels are 
generally among the largest occurring in any tissue. 
It sometimes happens that when a pitted or other vessel has 
lost its fluid contents, the neighbouring parenchymatous cells 
push bladder-like portions of their membrane through pores which 
are then formed in its wall, and then multiply by division and 
form a cellular mass which may cowpletely fill it—to this intra- 
cellular tissue the name of tyloses or thyloses has been given. It 
may be well observed in the wood of the Oak, in that of Robinia 
Pseud-acacia, in Periploca, and in the stem of Cucumis sativus. 
b. Spiral Vessels.—This name is applied to vessels with 
tapering extremities, having either one continuous spiral fibre 
running from end to end, as is commonly the case (fig. 104), or 
two or more fibres (jig. 105) running parallel to one another. 
Those with only one spiral fibre are sometimes termed Simple 
Spiral Vessels; those with more than one, Compound Spiral 
Vessels. The latter kind are well seen in the stem of the Banana 
and other allied plants, in the young shoots of the Asparagus, 
and in the Pitcher Plant. The fibre contained within the spiral 
vessel is generally so elastic as to admit of being uncoiled when 
the vessel is pulled asunder, in which case the wall is ruptured 
between the coils. This appearance may be commonly seen by 
the naked eye by partially breaking the young shoots, flower- 
stalks, or leaf-stalks of almost any plant ; or the leaves of the 
