1 54 MORPHOLOGY OF MEMBERS. 



among Lycopodiacese the genus Pst7o/um, among Orchideae Epipogum Gmelini and 

 Corallorhiza mnafa, 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 present even in the young embryo which proceeds from the 

 fertilised ovule (but not in Orchideae) ; it appears at the posterior end of the em- 

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

 and soon dies, as in Cryptogams and Monocotyledons \ or whether it continues 

 to grow more vigorously, like the rest, 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 enormously more numerous than the 

 primary roots, and of much greater importance to the plant, a special name is 

 superfluous wher^ the contrast to the primary root is not of importance. They 

 arise in the interior of the primary or secondary roots, and on stems and 

 petioles. The primary root with its secondary roots, or any root with its lateral 

 roots, may be termed a Root-syslem. 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 or tubers. 

 In Tree-ferns the stem is often densely covered throughout its whole length with 

 a felt of delicate roots. 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 

 petioles, as, for example, in Nephrodium Filix-mas, Asplenium Filix-foemina, Cera- 

 topteris thalidroides, &c. ; sometimes the fronds put out roots as in Mertensia 2. 

 When the stem possesses clearly developed nodes and internodes, the roots usually 

 spring from the former ; thus, for example, exclusively from the nodes in Equise- 

 taceae, and most commonly so in Grasses. 



Roots owe their origin either to the primary meristem, or to partially diff"erentiated 

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

 pletely diff'erentiated. The primary roots of embryos arise from quite undiff"erentiated 

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

 shown, originate near the growing point of roots, where the differentiation of their 

 tissues first begins; and with Phanerogams the same is the case. But stems may 

 also produce roots near their growing point, where the differentiation of the primary 

 meristem first commences; this occurs in the case of the creeping stems of Rhizo- 



^ [On the primary root of Monocotyledons, which disappears at an early period, see Falkenberg, 

 Vergleichende Untersuchungen iiber den Bau der vegetationsorgane der Monocotyledonen. Stutt- 

 gart 1876.] 



^ A leaf of Phaseolus multijlorus cut off at the pulvinus and placed in water developed from the 

 callus an abundant root-system, and remained living for some months. [The leaves oi Ficus elastica 

 behave in the same way.] According to Van Tieghem, the cotyledons of the sunflower, scarlet 

 runner, Cucurhita maxima, Mirabilis Jalappa, &c., when laid on damp moss in a temperature of from 

 22° to 25° C, produce in a few days a number of roots ; and this takes place even if the cotyledons 

 are cut into small pieces, the roots then proceeding from the sections of the vascular bundles. I have 

 myself seen a seedling of Cucurbita covered up too thickly with earth put forth long roots from its 

 cotyledons. See further Dodel, Jahrb. fur wiss. Bot. vol. VIII. p. 177. 



