142 EXTERNAL CONFORMATION OF PLANTS. 



vessels being formed nearer to the circumference of the bundle, whereas the later 

 bundles are always formed further inside, and hence centripetally in reference to 

 the diameter of the root. Where bast-bundles occur, they arise in the cavities 

 between the primary vascular bundles at the circumference of the fibro-vascular 

 mass (Fig. 116, p. 146). 



Although roots are generally distributed among vascular plants the higher 

 Cryptogams and Phanerogams, there occur even in these groups single species 

 from which they are entirely absent ; thus among Rhizocarpeae the genus Salvinia, 

 among Lycopodiaceae the genus Psilotum, among Orchideae Epipogiiim Gmelbii and 

 Corallorrhiza imiata are destitute of roots ; the little Lemna ( Wolffia) arrhiza does 

 not form roots, and is at the same time destitute of vascular bundles. 



With reference to the place of their formation roots are remarkably variable ; 

 a root is usually formed even in the young embryo which proceeds from the 

 fertilised ovule (but not in Orchidece). It appears at the posterior end of the em- 

 bryonal stem, and may be termed generally the Primary Root, whether it remains 

 weakly and soon dies, as in Cryptogams and Monocotyledons, or whether it 

 continues to grow more vigorously, like all other roots, as in many Dicotyledons. 

 But besides the primary roots, there are usually formed in addition a large number 

 of Secondary Roots, or simply Roots (since they are a thousand times more nume- 

 rous than the primary roots, and are also of much greater importance to the plant, 

 it is superfluous to denote them by a special name, where the contrast to the 

 primary root is not required). They arise in the interior of the primary or 

 secondary roots, and on stems and leaf-stalks. The primary root with its secondary 

 roots, or any root with its lateral roots, may be termed a Root-system. With the 

 exception of many Dicotyledons with a persistent strongly developed primary root, 

 the majority of roots spring from stems, especially when these latter creep, float, 

 climb, or form bulbs and tubers. In Tree-ferns the stem is often densely covered 

 wdth a felt of delicate roots throughout its whole length. In Ferns with densely 

 crowded leaves in which no portion of the surface of the stem is left bare, the 

 roots spring exclusively from the leaf-stalks, as, for example, in Aspiditnn Filix- 

 mas, Aspkniiwi FiIix-/oe??ima, Ceratopteris thalictroides, &c. ; sometimes the fronds 

 put out roots {e.g. Mertensia)^ When the stem possesses clearly developed nodes 

 and internodes, the roots commonly proceed from the former; thus, e.g. exclusively 

 from the nodes in Equisetacege, and most commonly so in Grasses. 



Observation of the nature of the tissues out of which the roots spring shows 

 that they owe their origin either to the primary meristem, or to partially differentiated 

 masses of tissue, or finally to a secondary meristem enclosed between layers completely 

 differentiated. The primary roots of the embryos arise from quite undifferentiated 

 primary meristem ; the lateral roots of Cryptogams, as Nageli and Leitgeb have 

 shown, originate near the piinctum vegetalionis of growing roots, where the differentia- 

 tion of their tissues first begins. With Phanerogams the same is the case, but stems 

 may also produce roots near their punctum vegetalionis, w^here the differentiation 

 of their primary meristem first commences ; this occurs in the case of the creeping 



^ A leaf of Pha&eohis mnliiflorui cut off at the pulvinus and placed in water developed from the 

 cortex of the intersected pulvinus an abundant root-system, and remained living for some months. 



