14 



LECTURE II. 



generally inconsiderable, since they find the nutritive fluid in abundance, while 

 the leaves, floating on the water or surrounded by the damp air, use up but 

 little water ; plants living in humus and root-parasites which have no green 

 foliage leaves, as Neottia, Lafhrcea, Orohatiche, have also but an insignificant 

 root-system in proportion to their body-weight, because in the absence of green 

 leaves the most important transpiratory surface is wanting. In general the root 

 fibres are the thinner, the more numerous they are in the root-system ; since, in the 

 formation of roots, it is essentially a matter of a large surface, with least possible 



mass of the organ. Hence 

 the less numerous, short 

 roots of plants devoid of 

 chlorophyll, as well as of 

 water-plants, are remark- 

 ably thick and fleshy; while 

 the young absorbing roots 

 of large trees and shrubs 

 ^■ery often scarcely attain 

 the thickness of a horse- 

 hair. If we now examine 

 a single root fibre more 

 closely, we have at its free 

 end the growing point, out 

 of which the whole cellular 

 tissue of the fibre becomes 

 developed. This growing 

 point is moreover encased 

 with a cap of firmer per- 

 manent tissue. The fore 

 end of the root is, in fact, 

 destined to be pushed for- 

 wards in the earth, much 

 as the point of a nail which 

 is driven into a board. It 

 is obvious that the solid 

 root-cap, with its smooth 

 slippery surface and its coni- 

 cal form, facilitates the pro- 

 gress forwards, and at the 

 same time protects the delicate tissue of the growing point. In aerial and aquatic 

 roots, which we may regard as derived or even reduced forms, the root-cap 

 is feebler, and is only in general an organ of protection against the drying up 

 of the growing point, or against its immediate contact with w^ater. The anatomical 

 relations need only be briefly referred to: each root-fibre consists of a central 

 or axial vascular bundle, or fibro-vascular cord, and a soft cortex of ex- 

 ceedingly thin-walled parenchyma cells surrounding it. The axial cord, the 

 histological constitution of which diff'ers considerably from the vascular bundles 



FIG. 6.- 



iigitudii 



of the : 



of a youngf A)tgiopteris 

 eziecta. Above, the youngest leaves (*) are still completely enveloped in the 

 stipules (ii b) ; st stalk of an unfolded leaf, with its stipules lib; n everywhere 

 the leaf-scars on the basal portions yy from which the leaf-stalks have been 

 separated ; cc the commissures of the stipules in longitudinal section ; iinu roots 

 (nat. size). 



