Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/97

] or as small canals when the cell-wall is very thick. Such cells are termed porous or pitted or dulled cells (fig. 21). In old cells, after the protoplasm has disappeared, the portion of the cell-wall which remained thin is often absorbed, and thns there is a true perforation of the cell-wall. These perforations often occur in groups both upon the cell- wall and upon the septum between superposed cells, and give rise to a remarkable sieve-like structure, in which case they are termed sieve cells The latticed cells of some authors are of a similar nature. When superposed porous or sieve cells coalesce by complete obliteration of the septum, then a pitted vessel, sievo tube, or duct is formed (fig. 22). These ducts aro usually of a larger size than other vessels ; they are well seen in the inner phloem layers and in the wood of trees, and they constitute the large rounded openings which are seen in the transverse section of the stems of the Oak, Poplar, Willow, &c. They also abound in the Bamboo (fig. 23), and in other plants of rapid growth. The names of bothrcndtyma and taplirenchyma have been given to a tissue composed of such cells. Not unfrequently contractions are visible on the outside of the vessel (fig. 22), indicating its formation by coalescence of superposed cells. To vessels exhibiting contractions of this kind, whether spiral or pitted, the terms moniliform and vermiform have been applied ; and the tissue composed of these moniliform vessels has been denominated phleboidal. In the ducts of many plants a remarkable appearance is produced by the protrusion, through the perforations into the cavity of the vessel, of portions of the adjoining cells, or, before its absorption, of the portion of partition-wall closing the pit. These portions appear as cells filling the interior of the vessel, and are described under the name of tylosis (fig. 24). It is well seen in the Walnut, Chestnut, Oak, &c. FIG. 21. -Porous or pitted coll from the Mistleto. FIG. 22. Moniliform dotted or pitted vessel from the Melon. FIG. 23. Section of a Bamboo, showing an angular network of cells, and the round apertures of pitted vessels. Fio. 24. Longitudinal section of the stem of a species of VedaiHJuyIantctaa-tn), showing tyln. -is in pitted vessels, a. Fir,. 25. Branching and anastomosing laticifcrous vessels The arrows mark the direction of the current

Laticifcrous vessels (fig 25) consist of long branching tubes or passages, having a diameter of about yj^th of an inch, forming, by their union, an anastomosis or network, like the veins of animals. They are the milk vessels and the proper vessels of old authors. They receive their name from containing an emulsion called latex, of a granular nature, often milky or coloured. They are seen in the India- rubber and Gutta-percha plants, the Mudar plant, the Cow-tree, Spurges, Dandelion, Lettuce, Chicory, and Celan dine, frequently containing a large quantity of caoutchouc. Usually these vessels are thin-walled, but sometimes slightly thickened. They are found most abundantly in the phloem layers rarely in the xylem or wood layers (Papayacese). They are the result of the coalescence of anastomosing or rectilinear rows of cells, and sometimes they seem to have resulted from the conversion of other vessels. In some Araceae they seem to represent spiral vessels. In Asclepia- daceue they are evidently bast-fibres. Some consider them as merely intercellular canals. The milky sap of Euphorbia phosphorea is said to be luminous. The latex exhibits movements which have given origin to the name cinen- chyma applied to laticiferous tissue by some authors. Those movements, classed under the name cyclosis, must not be confounded with the motion of protoplasm in cells which is designated rotation.

We have seen that the cellular tissue is sometimes in complete, that is, the cells do not touch on every side (fig. 9). The intervening spaces are called intercellular spaces, and these may be either circumscribed cavities called lac^mce, or they may extend for some length through the tissue as intercellular canals ; but these two structures pass into one another. In the earliest stage of development the tissue is always complete, and these spaces are formed subse quently by a splitting of the partition or common wall of the cells, and they may subsequently be increased in size by an absorption of the investing cells. These lacunae and canals may contain air, especially in aquatic plants, to give them buoyancy, as in Potamogeton (fig. 26), or they may be receptacles for various secretions, and when they exist as canals they usually aid in conducting sap. The intercellular canals are exceedingly well seen in coniferous plants, where they constitute resin passages, forming a continuous system throughout the plant, and arranged at intervals in concentric circles in the xylem or wood portion of the stem. Fig. 2(3. Vertical section of the leaf of Potamogeton or Pondweed, showing air cavities or lacunoj 7, and purenehymatous cells p, with granules.

Chlorophyll is the green colouring-matter of plants. It Substances occurs in the cells of the superficial parts of plants united found iu with small portions of the protoplasm (chlorophyll bodies), cells&amp;gt; which are combined into grains of various forms. Starch- grains are usually abundant in the chlorophyll bodies. Chlorophyll is soluble in alcohol and ether. It consists of four substances, two yellow and two green, which possess distinct optical properties. It gives a black band in the red of the spectrum. Physiologically it is very important. It is developed under the action of light, and undergoes changes according to its state of oxygenation. Hence the varied tints of leaves in autumn. Numerous colouring matters occur in plants, especially in flowers, and all such when not green arc included under the general term chromule. Starchy and oily matters and albuminoids occur very abundantly in the cells of plants, where they are stored 