Structural Botany : The Root. IX- 



The Primary Hoot of a seedling (e. g. Bean) appears as the direct continuation 

 of the primary axis of the shoot, passing down into the soil, as a main Tap-root, 

 with definite growing Apex, and lateral ramification ; the laterals spread more 

 horizontally, and again branch to a highly ramified system penetrating the soil in all 

 directions. 



Ordinary Soil is a very special material, of inorganic particles and colloidal 

 organic de*bris, with spaces containing air and water, as a complex organization 

 physically and chemically ; and also biologically as containing races of micro-organisms 

 (Bacteria and Protozoa). The essential part of soil may be preferably regarded as 

 a living complex of bacteria engaged in breaking down decomposing plant-residues, 

 largely of cellulose origin, as humus, to which inorganic particles are largely accessory, 

 One gram of good soil may contain anything from 10-50 millions of Bacteria. For 

 successful penetration by roots the material requires to be loose in texture, well-aerated, 

 with considerable water-content, as facts of general horticultural experience. Land- 

 plants are normally furnished with ' soil-roots ' : ' water-roots ' and ' air-roots ' involve 

 subsidiary problems. 



The essential function of the root is Absorption of solutions in the soil ; the 

 mechanism of the process, being complex, involves a stationary habit ; fixation of the 

 plant to the ground, as adaptation to resist uprooting effect of wind, &c., is quite 

 secondary. Normally the connexion has become so intimate that adult plants with 

 less power of regenerating new root-systems are with difficulty ' transplanted'. The 

 solutions in the soil vary enormously, but are normally extremely dilute ; inorganic 

 solutes being largely ionized. Essential substances are determined by 



Culture Solutions, experimentally. A typical laboratory medium may contain 

 Potas. Nitrate, i g., Sod. Chlor., Calc. Sulph., Mg. Sulph., Calc. Phosphate, each -| g., 

 and a trace of Iron salt; or about 3 g. of salts to a litre (1000 g.) Sea-water, 35 g. 

 per litre, is over ten times as strong; lake- and river-water about -^fa the latter. 

 The osmotic value of sea- water is 23 atmospheres, that of rain-water may be nil; 

 soil waters range between. Absorption can only take place at the peripheral layer of 

 cells in direct contact with the medium ; and in typical soil-roots the units of this layer 

 are produced into special filamentous growths presenting greatly increased surface, as 

 1 Root-Hairs' ; these may attain a length of 1-5 mm., with a diam. of 6-30 /*. The 

 layer producing them is known as the ' Piliferous Layer ', as quite distinct from the 

 conception of the epidermis of the aerial shoot-portion. 



A typical Boot-Hair extends as a dilatation from a localized area of the original 

 cell-wall, with protoplasm, aqueous contents, and nucleus controlling its extension ; 

 the plastic apex pressed between particles of soil may become irregular in shape, and 

 contains little cellulose. Such units have usually but a short life, and die away as new 

 ones are continually produced at the growing apex ; e. g. on pulling up a plant the 

 root-hairs are usually broken off, and remain in the soil (cf. Groundsel, root-hairs 

 10 /A diam.). 



Boot Apex : The root increases by a terminal growing-point, essentially similar 

 to that of the stem, and presenting similar differentiation into distinct layers, con- 

 ventionally distinguished as calyptrogen, periblem, and plerome. The calyptrogen is 

 many-layered, and constitutes a definite ' Root-cap ' region, giving the apex a pointed 

 conical appearance, the older layers of which are gradually worn away by friction in 

 passing through soil. The initial layers are internal, marking a dome-shaped curve 

 as the true growing point. The periblem (or cortical region) in some cases (Zea) 

 reduces to a single row of cells at the actual apex, giving very distinct layering ; in 

 others (Bean) it is ill-marked. The central region (plerome) gives rise to the con- 

 ducting stelar tract. The calyptrogen is either entirely worn away as root-cap (Zea), 

 or leaves one layer only (Dicots.) ; in the latter case this becomes the piliferous layer ; 

 in the former, root-hairs are produced from ' periblem '. Such discrepancies show that 

 apical layering is of no strict morphological value, but may characterize minor groups 

 as interesting variations on a general physiological mechanism. 



Zones of Growth are more condensed than is usual in the case of typical 

 stems. The second ' zone of elongation ' is but short, giving increased power to the 

 thrust behind the dividing meristem of the apex ; and this region is again responsible 



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