The stereom is furnished either by cortical cells or by the tracheal elements, in a few cases by fibres which arc probably homologous with sievetubes.
Aquatic larvae with caudal leaf-gills or with rectal tracheal system.
During the process the thin walls are stretched and the turns of the spiral become pulled apart without rupturing the wall of the tracheid or vessel, If the pitted type of tracheal element were similarly stretched its continuously thickened walls would resist the stretching and eventually break.
In other cases the epithem may be absent altogether, the tracheal strand debouching directly on the lacunae of the mesophyll.
The nymphs of the Perlidae are closely like their parents and breathe dissolved air by means of tracheal gills on the thoracic segments, for they all live in the water of streams. They feed upon weaker aquatic creatures, such as the larvae of Mayflies.
Prosoma and opisthosoma; a, tracheal stigma; b, last somite.
In most respects, the shortened abdomen, for example, they are more specialized than the Thysanura, and most of the features in which they appear to be simple, such as the absence of a tracheal system and of compound eyes, can be explained as the result of degradation.
In the secondary tissues of Dicotyledons we may have, as already described, considerably more differentiation of the cells, all the varieties being referable, however, on the one hand to the tracheal or sieve-tube type, on the other to the parenchyma type.
External tympaniform membranes exist, with great variations, between the specialized one or two last tracheal and some of the first bronchial rings.
Omitting the paired tracheo-clavicular muscles, we restrict ourselves to the syringeal proper, those which extend between tracheal and bronchial rings.