The structure of the above forms of tissue may be best understood in relation to their development. It has been stated that the essential part of the cell is the protoplasm. As cells grow older, new matter is deposited by the protoplasm upon the inner surface of the cell-wall, either to a small extent, evenly and uniformly, as in ordinary parenchyma, or unevenly, in the form of spiral layers, forming fibres or bands, leaving bare spaces, where the original cell-wall exists alone. The matter thus deposited is called secondary deposit, the original cell-wall being the primary deposit. When the secondary deposit covers the interior of the cells except at certain slit-like spaces, we have the appearance figured in Pl. I. fig. 6 _a_. When the deposit forms a spiral fibre, or a series of rings, we have the spiral or annular vessel or duct. And when the interspaces between the coils of a close spiral fibre are filled up except at certain spots, we have the dotted or reticulated vessel or duct. In many instances, these deposits are present together: thus, sometimes the outermost deposit leaves rounded pits or dots, while an inner portion forms a spiral fibre (Pl. I. fig. 11 _b_); or one layer leaves simple rounded pits, while the other leaves smaller slits or dots placed opposite the former (Pl. I. fig. 6 _b_). In some cells the cavity is almost entirely filled up by secondary deposit, which leaves minute canals radiating from a small cavity in the centre to the circumference, as seen in the transverse section of a plum-stone (Pl. I. fig. 56); here the canals appear as dark lines. In others, again, the secondary deposit forms several distinct layers, leaving channels very similar to those of the last; an example is met with in the gritty tissue of the pulp of a pear. The obvious use of the pits and channels in the above tissues is to preserve the permeability of the walls of the elements, which would be destroyed if the walls were equally thickened all over. _Cell-formation._--New cells are formed by the division of old or parent cells. The actual process of division is difficult to observe, as it requires prolonged observation; but cells are often met with in all stages of division, of which some instances will be pointed out hereafter. The cell-division takes place in two ways, either according to the endogenous (ἔνδον, within, γεννἁω, to produce), or the exogenous (ἔξω, outside, γεννἁω) method. The manner in which the division takes place in the former is this:--At first a slight indentation or constriction of the protoplasm occurs at the line of division; this deepens until the protoplasm is completely divided. The freshly divided surfaces then become coated with a new portion of cell-wall, so as to make two or more new cells, which either remain in contact or separate from each other. In some cases, the divided portions of protoplasm become coated all over with new cell-walls.