Ill 



ORGANOGRAPHY. 



A 



Fig. 227. 



to that of a monocotyledonous stem. Where they break through 

 they are surrounded at the base by a kind of sheath or collar called 

 the coleorhiza {fig. 225, co). They also grow by additions within 

 their extremities like true roots, and are terminated like them 

 by what has been termed a pileorhiza. In the Screw Pine, and 

 in some other adventitious aerial roots, the pileorhiza may be 

 well seen in the form of a calyptra or cap-like covering at the 

 extremity of each root. The pileorhiza of a monocotyledonous 

 root, like that of a true root, is commonly thrown oflf as develop- 

 ment takes place behind it ; but in certain aquatic plants, as 

 in the Duckweed {fig. 226), it is persistent, and appears in the 

 form of a long sheath over the end of the root ; and is continually 

 pushed onwards by the development of the cells within the apex. 

 Some botanists regard this structure as diiferent in its origin 

 and characters to the ordinary pileorhiza; in fact, they limit the 

 term pileorrhiza to it. 



The adventitious roots of Dicotyledonous plants arise in a 

 somewhat similar manner to those of Monocotyledons, making 

 their first appearance as little conical bodies in the neighbour- 

 hood of the cambium layer, and ultimately breaking through 

 the bark and appearing on the 

 surface. They also grow by additions 

 within their extremities, and each is 

 protected by a pileorhiza, and has 

 at its base a coleorhiza. They haA'e 

 under ordinary circumstances a simi- 

 lar structure to that of true roots. 



Adventitious roots generally, like 

 true roots, have no leaves or buds, 

 and when subterranean, have no 

 epidermis furnished with stomata; 

 hence when derived from Dicotyle- 

 dons, they are distinguished from 

 the stem by the same characters as 

 the true root. The adventitious roots 

 of Monocotyledons and Acotyledons 

 have a similar structure to their re- 

 spective stems, as will be afterwards 

 noticed ; but in other respects, when 

 exposed to similar influences, they 

 present the same distinctive cha- 

 racters from the stem as other roots. 

 Aerial roots are, however, from their 

 point of union betweenTtem exceptional position, frequently fur- 

 and root t The stem. /, /. nigged with a true epidermis and 

 Leaves. &, h. Buds. , , -^ , . ^ 



stomata, and are sometimes oi a 



green colour ; but in other respects they resemble other adven- 

 titious roots. 



Fig. 227. Lower part of the 

 stem and root of the common 

 Stock, r. The tap-root with 

 Its branches, c. The neck or 



