Oil the Siructure of Roots. 



471 



Fiff. 3. 



roots, are the only roots of Monocotyledons ; but this kind of root 

 is by no means confined to that class ; adventitious roots occur in 

 Dicotyledons also in every case 

 where the plant forms roots from 

 the stem, independently of the tap- 

 root of the seed, as in all cuttin<:;s, 

 and natural propagula of all kinds, 

 such as tubers, strawberry run- 

 ners, &c, 



A root therefore may be a true 

 descending axis, as it is sometimes 

 called, that is the prolongation of 

 the lower end of the stem ; but 

 roots are constantly formed from 

 the side of the stem, either near 

 its base or at or in the neighbour- 

 hood of the places Avhere leaves 

 are attached ; they break out from 

 beneath the rind of the stem, and 

 thus are surrounded at the point 

 of exit by a little ragged collar ; 

 also, when they decay and fall off, 

 as is commonly the case with those 

 formed underground, they leave a 

 scar-like hole at the place where 

 they emerged. Adventitious roots 

 are also developed from the sides 

 of tap-roots. 



The difference between true 

 tap-roots and adventitious roots is 

 not of great importance, although 

 they frequently give the character 

 to the general form and arrange- 

 ment of the root structures ; the 

 tap-root, forming the characteris- 

 tic branched descending axis of 

 the Dicotyledons, such as turnips, 

 peas and beans, flax, &c., is a 

 direct production of the lower end 

 of the stem. But in perennial herbaceous plants which are pro- 

 pagated by stools, or offsets, one of the adventitious roots of the 

 offset sometimes assumes all the ordinary characters of a tap-root, 

 as in the monkshood. 



The adventitious roots which supply the place of the tap-root 

 in Monocotyledons like grasses, corn-plants, onions, asparagus, 

 &c., by their number and equal size give a fibrous character. 



Koot of Shepherd's Purse. 



