= Solution
\b[<Stomata> are regulated pores of the plant epidermis; the <primordial utricle> is an older name for living peripheral cell contents; the <vascular cambium> is a tissue that produces secondary conducting tissues.] These are respectively an opening apparatus, part of a cell's living contents, and a region of dividing cells.
A <stoma> is an aperture bounded by two <guard cells>, commonly with associated subsidiary cells. Changes of <guard cell> <turgor pressure> alter the aperture. <Stomata> permit entry of <carbon dioxide> for <photosynthesis> and loss of <water vapour> through <transpiration>, while also allowing other gas exchange. Most leaves have many on their lower surface, but distribution varies, so absence from the upper surface is not a definition. A pore is not itself a minute leaf <blood vessel>.
The <primordial utricle> was distinguished in nineteenth-century microscopy as the protoplasmic layer inside the <cell wall>, especially conspicuous around the large central <vacuole> of a mature <plant cell>. In modern vocabulary this includes peripheral <cytoplasm> bounded externally by the <cell membrane>; the vacuolar boundary is the <tonoplast>. The old term should not be reduced to a synonym of the rigid <cell wall> or assumed to designate an independently living bag. In <plasmolysis>, an appropriate external <hypertonic solution> draws <water> from the cell and the living contents withdraw from the wall, making the distinction visible.<Biology 2e by Mary Ann Clark and others/5.2 Passive Transport>[Tonicity in Living Systems; plasmolysis and Figure 5.13]{ref}
The <vascular cambium> is a lateral <meristem>, characteristically situated between the <xylem> and <phloem> of a woody stem. Its divisions produce secondary <xylem> towards the inside and secondary <phloem> towards the outside; radial derivatives also contribute to vascular rays. Activity connects the growth of the conducting system with thickening of the axis. It is not simply a film of nutritious liquid that spontaneously turns into wood. The <cork cambium> is a different lateral <meristem>, concerned with the protective outer covering.<Biology 2e by Mary Ann Clark and others/30.2 Stems>[Secondary Growth in Stems: vascular cambium, secondary xylem/phloem and cork cambium]{ref}
The course of sap can now be followed from soil to growing tissue. Root cells acquire mineral ions, and <water> enters along a <water potential gradient>. Water and dissolved minerals move into the <xylem> and upwards through the root and stem to the leaves. In a transpiring plant, evaporation from moist internal leaf surfaces and diffusion out through <stomata> maintain a pull on the continuous water columns. <Cohesion> between water molecules and the mechanical support of xylem walls allow tension to be transmitted. <Root pressure> can drive exudation or guttation under some conditions, but it is not a sufficient universal explanation for supply to the tops of tall trees.
In leaves, <photosynthesis> incorporates <carbon dioxide> into organic compounds. Soluble products, especially <sucrose>, enter the <phloem> and move from sources to sinks, such as growing roots, buds, fruits or storage organs. Thus elaborated sap is not invariably a single downward stream: the direction depends on where a source and a sink lie. The modern <pressure-flow hypothesis> explains bulk movement through differences of <hydrostatic pressure> associated with loading and unloading solutes. Neither the ascending nor the assimilate stream forms an exact analogue of an animal's closed <blood circulation>.<Biology 2e by Mary Ann Clark and others/30.5 Transport of Water and Solutes in Plants>[Water Potential; Transpiration; Movement of Photosynthates: the cohesion-tension and pressure-flow accounts]{ref}
A short experimental history makes these distinctions concrete. In <Vegetable Staticks by Stephen Hales> (1727), <Stephen Hales> measured water use and perspiration by plants and attached tubes and pressure gauges to cut vines. Exudation into a sealed vertical tube demonstrates a positive pushing pressure generated below the cut. The corresponding observation after the bleeding season differed, showing that root pushing and the supply to a leafy, transpiring shoot need not have the same explanation. Weighing a potted plant while limiting evaporation from the soil provides another balance: water lost from the system must be compared with its supply and changes of storage.<Vegetable Staticks by Stephen Hales>[Experiments on perspiration; Chapter III, Experiments XXXIV–XXXV, pp. 100–103, vine-stump tubes and seasonal contrast]{ref} A cut shoot taking up water is informative, but its uptake alone does not establish all the forces acting in an intact rooted plant.
For the <phloem>, removal of a ring of bark can interrupt assimilate transport while leaving a substantial wood pathway intact; accumulation above the interruption distinguishes the two transport systems, although injury complicates any inference about a natural circulation. In an 1866 account, terms such as crude and elaborated sap could summarize these routes. The modern quantitative <water potential> and <pressure-flow hypothesis> accounts extend beyond that historical vocabulary.
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