Structural Botany : Stem Mechanism. VII. 



Differentiation of form of cells, texture of walls, and character of contents imply 

 differentiation of function. Names given to tissues are of value only in so far as they 

 express a specialization of parts in a complex mechanism. Details of microscopic 

 observation are the chief method of deducing the meaning of the organization; 

 e.g. What each unit does, how it is done, and on what evidence, are the essential 

 problems. 



The Epidermis secretes the cuticle as a more or less impermeable film restrict- 

 ing loss of water from the entire external surface ; Collenchyma affords mechanical 

 cohesion under wind-strain : the ducts secrete terpene-derivatives, but are of little 

 significance in this type (cf. Ptnus). 



The general Cortex is photosynthetic as far as light can penetrate, and acts as 

 a ground-tissue of units apparently not required for any more special function : the 

 endodermis controls lateral access to and from the stele. Pericyclic strands of fibres 

 afford mechanical cohesion, strength, and elasticity, to the axis in response to wind- 

 action. The Phloem includes tissues distributing synthesized ' food ' to non-photo- 

 synthetic regions, as soluble carbohydrate sugars and colloidal proteid ; the latter 

 requiring perforated sieve-apertures. The sieve-tubes, being destitute of a nucleus in 

 the functional condition, can have only a partial activity : the companion-cells, with 

 distinct nuclei, as sister-cells of sieve-tubes, apparently play a more active part, as in 

 storage and regulation. The Cambium is the meristem continually adding new 

 tissues, and implying that the older units become effete. Xylem includes tissues 

 devoted to the conduction of the transpiration-current. Pitted-vessels, with continuous 

 tube-cavity, formed by the obliteration of the original transverse septa, represent the 

 most highly efficient water-pipes; the bordered pits acting as countless valves per- 

 mitting the passage of water but not of air (cf. mechanism of pit-chamber, pit-membrane, 

 and torus). Spiral tracheides, universal in all vascular land-plants, as the first-formed 

 conducting units, merely present a spiral supporting internal thread of lignified 

 material on a cellulose wall, originally close-set. Annular tracheides, as variants, are 

 open to compression between the rings. Living parenchymatous cells, with osmotic 

 pressure of several atmospheres, are stronger than any dead wall unless thickened 

 solid. Medullary-ray parenchyma and pith are of the nature of ground-tissue, storing 

 starch, or dilating instead of dividing, and soon dying off in reduced supply of light, 

 water, and free oxygen. 



CUCURBITA (Vegetable Marrow) an interesting example of herbaceous stem ; 

 also annual in duration ; with more exact number of large V.B. in a 5-6 ridged shoot, 

 bearing enormous leaves (18 in. across), and huge fruits : the plant is a tendril-climber, 

 and the mechanical tissues are ill-differentiated ; but the conducting tissues (^ and <) 

 are as fine as may be found, and far more elaborate than those of Helianthus, in 

 correlation with the great photosynthetic and transpiring area, as also the amount of 

 food to be conducted to the massive fruits within a few summer months. But the 

 cambium-ring is never completed, the bundles remain isolated, and the medulla 

 breaks down leaving a cavity. 



Transv. sect, of the ridged stem shows V.B. in double series, corresponding to 

 the ridges and spaces alternating : the pitted-vessels of the metaxylem, -5 mm. diam., 

 can be seen with the naked eye. From the periphery inwards, note : Epidermis 

 little differentiated, with thin cuticle ; beneath it tracts of collenchyma, partial only, 

 a dozen cells deep (20 //,), with conspicuous cellulose thickening at the angles, giving 

 star-effects. Parenchymatous cortex of half a dozen rows : endodermis without starch, 

 and no special character ; but recognizable by being immediately outside a continuous 

 zone of pericyclic fibres, little thickened, 4-6 rows, and lignified. Within this zone 

 pericyclic parenchyma as a tract i mm. broad, of cells similar to med. rays, and in 

 older stems storing starch in cluster-groups. The Phloem contains large sieve-tubes, 

 to 70-90 fji diam., with associated conspicuous companion-cells: transverse sieve- 

 plates are seen in face-view, with netted pores (5 //,) and conspicuous proteid-contents, 

 irregularly coagulated, and staining brown with Iodine sol. Cambial cells are 30 //, 

 wide, giving very regular radial rows ; the latter much disturbed in the young 

 metaxylem by dilatation of young vessels. The Metaxylem of Pitted Vessels and 

 Pitted Tracheides shows no mechanical fibres, and but little parenchyma. The 



