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A hypertonic solution favors initial net water loss under the reference conditions above. In a walled plant cell, enough loss can produce plasmolysis.

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.^[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.^[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.^[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.^[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.

Plasmolysis is withdrawal of the membrane-bounded contents from parts of a cell wall after water loss. A sufficiently concentrated external hypertonic solution, with solutes effectively excluded by the cell membrane over the observation time, can cause this. The cell wall remains; the enclosed material contracts. Plasmolysis is therefore distinct from osmotic lysis, which breaks the cellular boundary during excessive swelling. A return to a less concentrated surrounding chemical solution can reverse plasmolysis if the biological cell remains functional.^[ref]

Tonicity describes how a surrounding chemical solution changes cellular volume through osmosis. It depends on which solutes remain effectively excluded on the timescale of interest, not simply on the amount of dissolved material. The following comparisons take a functional cell membrane, an initially unpressurized biological cell, and effectively excluded solutes as the reference; rising turgor pressure can oppose entry in a walled biological cell.

Operationally, hypotonic solutions cause sustained swelling, hypertonic solutions sustained shrinking, and isotonic solutions no sustained volume change in the specified passive comparison. State the reference biological cell, membrane properties and observation interval. A permeating solute can transiently change water potential while crossing; after its chemical potential equilibrates, its initial unequal concentrations cannot be treated as a permanently retained osmotic load. Thus equal osmolarities need not imply isotonic solutions. Conversely, a stationary walled biological cell may have unequal osmolarities balanced by turgor pressure.^[ref]