INTERNAL ANATOMY OF ORGANS. BOOTS. 



535 



Fig. 581. 



or rhizogenous layer opposite to the vessels (fig. 581 , A, # r), except in 

 Grasses, where they originate opposite the liber-bundles. The anatomy 

 may be 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 at first 

 wholly cellular, and we may dis- 

 tinguish in them three parts, a 

 woody axis, which soon becomes 

 continuous with the fibro-vascular 

 plexus ; a cortical parenchyma, con- 

 tinuous with the inner part of that 

 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 

 this way, it presents a circular free 

 edge standing up slightly like a 

 collar around the base of the free 

 part of the root : this is called the 

 coleorhiza (fig. 581, B, c) by some 

 authors. The conical hood upon the 

 apex of the root forms the root-cap 

 or the pileorhiza (fig. 581, B,;?), and 

 is more or less persistent in different 

 cases ; in aquatic plants it becomes 



greatly developed, as may be seen in /. f fn f 



the Duckweed (Letnna), where it 



forms a long sheath, appearing as Development of adventitious roots 



nium. A, B. Fragments of a rhizome with 

 cortical parenchyma (cp), fibrous layer (f) 

 where the fibro-vascular bundles terminate, 

 and central region (mp) in which the bun- 

 dles run. A, a r, shows an adventitious root 

 arising from the cambium tissue at the out- 

 side of the fibrous layer: in B the more ad- 

 vanced root (a r) has emerged, leaving a 

 ragged collar or coleorhiza (c), and haying 

 a root-hood or pileorhiza on its extremity . 



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 continual 

 development of cells just behind the 

 apex. 



The pileorhiza may be compared 

 to a kind of shield or guard to the 

 tip of the root, protecting the nascent tissue, by the expansion of which 

 it is pushed forward, itself always possessing a certain solidity which 

 enables it to penetrate between the particles of the soil. 



In a cross section of the root of a Monocotyledon we see the centre 

 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- 

 vascular bundles of the stem in the fibrous region. Secondary adven- 

 titious roots are formed in the same way in the roots, originating imme- 



