SCALAR! FORM VESSELS. 
101 
generally describe membrane as tearing irregularly, but 
the hairs found on the outside of the fruit of Cycas 
revoluta, which are of some considerable size, will, 
when separated from the fruit, exhibit the appearance 
shown in Fig. 87 ; both extremities are pointed, but it 
is the one attached to the fruit that unrolls spirally. 
I shall next draw your attention to the variety of 
vessel which, from its peculiar markings somewhat 
resembling the rounds of a ladder, is called Scalari- 
form ; but this, like the other varieties, is but a modifica- 
tion and later stage of existence of the true spiral vessel, 
resulting from an unequal deposit of the secondary 
matter upon the inner surface of the cell-wall. The 
parts in which there have been no deposits, or, in other 
words, the pores, are always more or less elongated, 
like a slit, and have both extremities rounded. If one 
of these vessels, when divided vertically, be examined 
with a power of at least two hundred and fifty 
diameters, the depth of the pits may be distinctly 
seen. 
These vessels present another peculiarity : they are 
often of hexagonal figure, and the markings, as well as 
the sides, are generally of uniform size, as shown 
in Fig. 88 ; in other cases, as in Fig. 89, both the 
sides and the markings are unequal. 
The scalariform tissue is best seen in the root of 
Ferns , on tranverse sections of which, the naked eye 
discerns rows of black dots, with intervals of a lighter 
colour ; the former are bundles of cellular and ligneous 
