The only pathways for the gases which thus pass between the cells of the mesophyll and the outside air are the stomata.
In the leafy shoot this function is mainly localized in the cortical tissue of the leaves, known as mesophyll, Mesophyli.
In fleshy leaves which contain a great bulk of tissue in relation to their chlorophyll content, the central mesophyll contains little or no chlorophyll and acts as waterstorage tissue.
The cells of these sheaths are often distinguished from the rest of the mesophyll by containing little or no chlorophyll.
Chiefly, they provide a number of channels, penetrating every part of the leaf, along which water and dissolved salts are conveyed to, and elaborated food-substances from, the mesophyll cells.
As a bundle is traced towards its blind termination in the mesophyll the peridesmic stereom first disappears, the sieve-tubes of the phloem are replaced by narrow elongated parenchyma cells, which soon die out, and the bundle ends with a strand of tracheids covered by the phloeotermic sheath.
These bodies, known technically as chioroplaIts, are found embedded in the protoplasm of the cells of the mesophyll of foliage leaves, of certain of the cells of some of the leaves of the flower, and of the cortex of the young twigs and petioles.
The whole arrangement has a strong resemblance to the lacunae, mesophyll and stomata, which form the assimilative and transpiring (water-evaporating) apparatus in the leaves of flowering plants.
The stomata are in direct communication with the ample system of intercellular spaces which is found in the loosely arranged mesophyll (spongy tissue) on that side.
These, with their associated Stelar stereom, form a kind of framework which is of great Tissueol Leaf and importance in supporting the mesophyll; but also, and R~t.
It is known as mesophyll, and is formed of two distinct series of cells, each containing the green chlorophyll-granules, but differing in form and arrangement.
The characters of leaves most useful for diagnostic purposes are the position of the stomata, the presence and arrangement of resin-canals, the structure of the mesophyll and vascular bundles.
A pine needle grown iji continuous light differs from one grown under ordinary conditions in the absence of hypodermal fibres, in the absence of the characteristic infoldings of the mesophyll cell-walls, in the smaller size of the resin-canals, &c. The endodermis in Pinus, Picea and many other genera is usually a well-defined layer of cells enclosing the vascular bundles, and separated from them by a tissue consisting in part of ordinary parenchyma and to some extent of isodiametric tracheids; but this tissue, usually spoken of as the pericycle, is in direct continuity with other stem-tissues as well as the pericycle.
The occurrence of short tracheids in close proximity to the veins is a characteristic of coniferous leaves; these elements assume two distinct forms - (I) the short isodiametric tracheids (transfusion-tracheids) closely associated with the veins; (2) longer tracheids extending across the mesophyll at right angles to the veins, and no doubt functioning as representatives of lateral veins.
It has been suggested that transfusion-tracheids represent, in part at least, the centripetal xylem, which forms a distinctive feature of cycadean leaf-bundles; these short tracheids form conspicuous groups laterally attached to the veins in Cunninghamia, abundantly represented in a similar position in the leaves of Sequoia, and scattered through the so-called pericycle in Pinus, Picea, &c. It is of interest to note the occurrence of precisely similar elements in the mesophyll of Lepidodendron leaves.
The leaves have a single main bundle, and in the mesophyll are four longitudinal series of large intercellular spaces separated by transverse diaphragms. The sporangia, which are situated singly on the adaxial surface of the leaves, between their insertion on the stem and the ligule, arise from a considerable number of epidermal cells.
The mesophyll of Pinus and Cedrus is characterized by its homogeneous character and by the presence of infoldings of the cell-walls.
Abies, Tsuga, Larix, &c., the mesophyll is heterogeneous, consisting of palisade and spongy parenchyma.
In the leaves of Araucaria imbricata, in which palisade-tissue occurs in both the upper and lower part of the mesophyll, the resin-canals are placed between the veins; in some species of Podocarpus (sect.