ROOTS. 



[ 667 ] 



ROOTS, 



derm tlian the stem. The roots of Dico- 

 tyledons increase in diameter by animal 

 layers of wood formed in the lihro-vaseular 

 bundles; these, however, aro much less 

 regular in their arrangement than those of 

 the stem on account of the tortuous course 

 of tlie roots ; hence, while the wood of the 

 roots is often useful for ornamental purposes, 

 it is comparatively valueless for carpenters' 

 uses. The branches of the axial root are 

 originally gTowths from the apex of the root 

 thrown oft to the sides as it were, and their 

 woody axis is derived from a division of 

 that of the main I'oot. Tlie radical of a 

 germinating Dicotyledon has its pileorhiza, 

 and grows, in the same way as that of the 

 Monocotyledons, by development of cells 

 just behind the apex. Young roots are 

 covered by a delicate epidermis ; and the 

 cells of this are abundantly produced into 

 hairs (fibrillfe) in many plants, especially in 

 those growing on light soils ; these fibrils 

 are deciduous, the delicate epidermis, which 

 is always destitute of stomata, being gra- 

 dually converted into a corky layer. Ad- 

 ventitious roots are very common in Dico- 

 tj'ledous, especially the herbaceous peren- 

 nial kinds, and they alone can exist on plants 

 raised from cuttings &c. of stems. The 

 roots originate much in the same way as 

 those of the Monocotyledons, appearing first 

 as cellular cones in the region adjacent to 

 the cambium -layer, with which the fibro- 

 vascular structure soon becomes confluent. 

 They break through the rind with a coleo- 

 rhiza, and protected by a pileorhiza, just as 

 in Monocotyledons ; but when once formed, 

 they appear to branch in the same manner 

 as the axial root, and not by the formation 

 of secondary adventitious roots. 



The radicle of a monocotyledonous embryo 

 is never developed ; but if a section is made 

 of the lower part of the embryo, we find one 

 or more little conical bodies imbedded in 

 the parenchyma ; these are the nascent ad- 

 ventitious roots, which soon appear exter- 

 nally, breaking their way through the 

 superficial tissue. The anatomy may, how- 

 ever, be more easily studied by tracing the 

 development of the adventitious roots on 

 the rhizomes of rushes, flags, and other 

 plants of this class. The roots originate in 

 the region where the fibro-vascular bundles 

 of the stem terminate, (and frequently form 

 a fibrous plexus). They are first wholly 

 cellular, and consist of three parts : — a 

 woody axis Avhicli soon becomes continuous 

 with tlie fibro-vascular plexus ; a cortical 



parenchyma, continuous with the inner part 

 of tliat of the parent stem ; and a kind of 

 conical hood of rather dense cellular tissue 

 enveloping the end of the root. As the 

 root grows it pushes the hood forward, 

 which breaks down the cellular tissue before 

 it, and finally appears externally. When 

 the epidermis is ruptured in tliis way, it 

 presents a circular free edge standing up 

 slightly like a coUar around the base of the 

 free part of the root : this is called the 

 coleorhiza by some authors. The conical 

 hood upon the apex of the root, called the 

 pileorhiza, is more or less persistent in dif- 

 ferent cases ; in aquatic plants it becomes 

 greatly developed, as may be seen in the 

 duckweed (Lenmd), where it forms a long 

 sheath, appearing as if slipped over the end 

 of the rootlet. The focus of development 

 of the root is within the pileorhiza, which is 

 pushed forward by the continued develop- 

 ment of cells just behind the apex. The 

 pileorhiza may be compared to a land of 

 shield or guard to the tip of the root, pro- 

 tecting the nascent tissue, by the expansion 

 of which it is pushed forwards, itself always 

 possessing a certain solidity, which enables 

 it to penetrate between the particles of the 

 soil. 



The centre of the root of a Monocoty- 

 ledon is occupied by prosenchymatous tissue 

 with a circle of vessels around it, the whole 

 enclosed by regular parenchyma, sometimes 

 by liber-cells and covered by an epidermis. 

 The ring of vessels spreads out into a kind 

 of rosette at the base, and anastomoses with 

 the extremities of the fibro-vascidar bundles 

 of the stem in the fibrous region. Secon- 

 dary adventitious roots are formed in the 

 same way in the roots, originating imme- 

 diately upon the vascular ring and breaking- 

 through the cortical parenchyma. The 

 woody adventitious roots of arborescent 

 ]Monocotyledons differ only in the greater 

 development of the fibro-vascular struc- 

 tures; and they emerge from the stem 

 (palms) in the form of thick conical shoots. 

 In the thickened adventitious roots of as- 

 paragus, which perform the function of 

 tubers, the parenchyma is greatly developed. 

 In the tuberous roots of Orchids the central 

 woody axis becomes ii-regularly expanded 

 into parenchymatous tissue, driving the 

 vessels out nearly to the periphery, so that 

 the characteristic structure is greatly dis- 

 guised. The aerial roots of the epiphytic 

 Orchids have the growing extremities 

 clothed by several layers of a parenchy- 



