or more ^or less hexagonal if 'packed'. A general term for such ordinary units 

 of construction is retained in < parenchyma', given by Grew from the fancied resem- 

 blance to air-sacs of lung-tissue. 



A Parenchymatous Cell, taken as a starting point, is a fairly isodiametric 

 living unit, with cytoplasm and one nucleus as the central control. All plant- tissues 

 are based on such a structure, or have been formed from them, however much they 

 may be modified for special functions : the parenchymatous cell is thus generalized in 

 metabolism and function, and aggregated units constitute a tissue as parenchyma. 

 In addition to the nucleus, the cytoplasm may present vacuoles with cell-sap, often 

 greatly distended until the plasma is left as a peripheral film, and the nucleus 

 is lateral : it is bounded by a wall of waste debris of metabolism, mainly 

 polysaccharide cellulose, so called from plant-origin, as a carbohydrate with 

 suggested complex construction (6 CHOH H 2 O) = C 6 H 10 O 5 of the type : 



OTOH^CHOH . CO - CHOH - CHOH - CH 2 (ring-formula, Cross and Bevan, 1918).. 

 The wall is deposited on the confines of the cytoplasm, in thinner or thicker series 

 of lamellae ; i. e. when thick, stratification may be noticed, while thin areas (pits) are 

 left for conduction of soluble substances between adjacent units, as also perforations 

 (more minute) for protoplasmic continuity. 



Living protoplasm in such cells may show movements of circulation in slender 

 strands (bridles), or a foam-like arrangement ; the peripheral tract (plasm a tic film) 

 is concerned more particularly with the regulation of osmotic mechanism. The 

 outer surface of the film may constitute a semi-permeable membrane, to which the 

 freely permeable cellulose-wall may act as a mechanical support : such a cell in water, 

 in virtue of solutes in the cell-sap, presents phenomena of differential diffusion- 

 pressure ; water can enter the cell, but the solutes do not pass out : an osmotic 

 pressure of 5-10 atmospheres, or more, is commonly established in ordinary plant- 

 cells, maintaining the protoplast in a state of turgidity. The mechanism is utilized for 

 the absorption of water, distension of the cell-wall, and growth ; such turgidity being 

 so far essential for plant-cells growing in air. On placing similar cells in a solution 

 of higher osmotic value, the action reverses, water leaves the cell-cavity, and the 

 protoplast retracted from the wall is said to be plasmolyzed. 



Nob t all cell-mechanism was originally evolved in the sea for metabolism in 

 a circumambient food-solution of 3| % total salts, and later transferred to air with 

 only atmospheric gases and aqueous vapour outside. Hence complications and 

 readaptations. 



Other cell-contents include plastids, as tracts of cytoplasm for special functions, 

 with a certain individuality of their own : the most general case that of the Chloroplast, 

 coloured green with chlorophyll, and of essential significance in photosynthesis : also 

 vacuoles with oil drops, crystals as calcium oxalate, tannin, pigments, &c., as the 

 commonest products of metabolism. 



As good example of cell-detail, cf. blue hairs on stamens of the flowers of 

 Tradescantia (available till first frosts) : threads (4 mm.) of 20 cells, or so, as oval 

 units, in single series, up to 300 ^ long, to globular at 75 /A diam. distally : optimum 

 200 p, with thin cellulose walls, and violet-blue cell-sap ; the cytoplasm colourless, as 

 a granular parietal layer, with slender bridles across the cell-cavity ; the coloured cell- 

 sap filling vacuoles ; nucleus (20 /x) central or parietal. Healthy cells show circulation 

 of protoplasm in the bridles and around the wall, as active streaming movements ; 

 large granules at 3 /x per sec., smaller at 6-7 ^ per sec. Plasmolysis, by 5% salt- 

 solution, causes the plasma to retract from cell-wall in places ; ultimately to a wholly 

 free spheroidal mass, with sap-concentration to deep violet colour. Washing with 

 water will restore full turgidity, and the circulation of living plasma will be seen to be 

 unaffected. Addition of spirit kills the cell instantaneously; the plasma being 

 coagulated irregularly, and the nucleus taking up the pigment that was in the cell-sap 

 after death. 



Less satisfactory phenomena of circulation may be noted in the epidermis 

 skinned from the inner scales of Onion-bulb ; but special interest centres in the fact 

 that these cells are units of a definite parenchymatous tissue-system. 



Elodea : In elongated cells of leaf-midrib, cytoplasm streaming along the walls 

 takes chloroplasts with it at 10 /x per sec. 



