Though the primary mass is so fernlike in appearance the larger tracheids show series of bordered pits, as do most of the tracheids of the Pteridosperms, in which they show a Gymnosperm-like character.
If the force acts parallel to the axis of growth, the tracheids are more likely to be displaced if the marginal cells of the medullary rays are provided with weak walls that are readily compressed.
Although the tracheids have their permeable portions or pits in their walls, liquids cannot pass through them with the greatest ease.
In pines and spruces the cells of the upper and lower rows of each tier or pith ray have "bordered" pits, like those of the wood fibre or tracheids proper, but the cells of the intermediate rows in the rays of cedars, etc.
These tracheids are characterized by the presence of peculiar pits upon their walls, best seen when thin longitudinal sections are made in a radial direction.
The pits in the radial walls of the ordinary xylem-tracheids occur in a single row or in a double row, of which the pits are not in contact, and those of the two rows are placed on the same level.
The roots of many conifers possess a narrow band of primary xylem-tracheids with a group of narrow spiral protoxylem-elements at each end (diarch).
When tracheids occur in the medullary rays of the xylem these are replaced in the phloem-region by irregular parenchymatous cells known as albuminous cells.
In the wood of Cypressus, Cedrus, Abies and several other genera, parenchymatous cells occur in association with the xylem-tracheids and take the place of the resin-canals of other types.
But the combination of smooth tracheids with small pits (subsection Paracembra) Pinus shares with Picea, Larix and Pseudotsuga.
Of other wood-characters, the presence or absence of tangential pits in the tracheids of the late wood establishes a distinction between Soft and Hard Pines.
Ray-tracheids with dentate walls and ray-cells with large pits are peculiar to Pinus.
Walls of the tracheids of the medullary rays dentate.
These combinations are Ray-tracheids with smooth walls.
Tracheids of the medullary rays with smooth walls.
Picea-like characters of the medullary rays--tracheids with smooth walls combined with the thick walls and small pits of the ray-cells.
The walls of the ray-tracheids may be smooth or dentate; the pits of the ray-cells may be large or small.
Walls of the tracheids of the pith ray with dentate projections.
If further assurance is desired, microscopic structure must be examined; in such cases reference has been made to the presence or absence of tracheids in pith rays and the structure of their walls, especially projections and spirals.
In some, as ash, the tracheids are wanting; apple and maple have no woody fiber, and oak and plum no fibrous cells.
Spruce is hardly distinguishable from fir, except by the existence of the resin ducts, and microscopically by the presence of tracheids in the medullary rays.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "tracheids" in a variety of sentences. We hope that you will now be able to make sentences using this word.
