H, Two adjacent cells (leptoids) of a food-conducting strand in Fucus (a Brown seaweed).
K, Optical section of two adjacent leptoids of the Moss Polytrichum juniperinum.
The leptoids are living and nucleated.
0, Optical section of two adjacent leptoids (sieve-tube segments) of Pteridophyte, with sieve plates (s.
Associated with the leptoids are similar cells without swollen ends and with thicker cross-walls.
As the aerial stem is traced down into the underground rhizome portion, these three mantles die out almost entirelythe central hydrom strand forming the bulk of the cylinder and its elements becoming mixed with thick-walled stereids; at the same time this central hydromstereom strand becomes three-lobed, with deep furrows between the lobes in which the few remaining leptoids run, separated from the central mass by a few starchy cells, the remains of the amylom sheath.
I, 0.) is also an elongated cell, with a thin lining of protoplasm, but destitute of a nucleus, and always in communication with the next cell of the leptom strand by perforations (in Pteridophytes often not easily demonstrable), through which originally pass strings of protoplasm which are bored out by a ferment and converted into relatively coarse slime strings, along which pass, we must suppose, the organic substances which it is the special function of the leptoids to conduct from one part of the plant to another.
In the primary and secondary tissue, is that the proteid cells of the phloem are here always sister-cells of the leptoids and are known as companion-cells.