On the Structure of Roots. 479 



pearing down to one bud or several buds collected together, situ- 

 ated at the upper end of a tuft of fleshy, tuberous roots. The 

 dahlia affords a remarkable example of tliis, as also the peony 

 and other garden plants. 



It is a general character of the class of plants called Mono- 

 cotyledons, that the radicle of their seeds is never developed 

 into a tap-root ; hence they always present fibrous roots, which 

 more or less completely resemble externally those of Dicoty 

 ledons. If we observe the germination of the seed of barley 

 (fig. 7), we find, instead of an elongating radicle, several equal, 

 thread-like filaments emerffins: from the seed-coats, havins: their 

 origin just above the point of the radicle. The same may be 

 observed still more clearly in the onion, since there the coty- 

 ledon rises up as a green leaf, and carries the seed-coat with it, 

 leaving the little blunt-ended stem with its tuft of root fibres 

 on the ground. If we follow the growth of the onion, we observe 

 that the stem does not grow up, but forms a wider base for the 

 successively developed leaves, the sheathing bases of which form 

 the bulb ; the tuft of roots is produced by the successive increase 

 of a number of new filamentous roots, always outside the old 

 ones : whether we leave the bulb in the ground, or take it up, 

 this central bunch of roots dies after the first season, and when 

 the bulb sprouts again, the new roots spring out outside the scars 

 of the old ones, and thus form a circular group. The same thing 

 is still more evident on the bulbs of the hyacinth. All these 

 roots of Monocotyledons are of the kind which we have called 

 adventitious. This adventitious character is more clearly dis- 

 played in the numerous cases where Monocotyledonous plants root 

 at the nodes or knots, as in creeping grasses, in the lower joints 

 of Indian corn, and most aquatic grasses, &c. The roots of 

 Monocotyledons are for the most part soft and fleshy, and gene- 

 rally present the form of long threads or cords of equal thickness 

 throughout, which, when they branch, give off the branches at 

 right angles. The roots of Dicotyledons branch, like their stems, 

 by shoots, which are graduated in size from end to end, running 

 off to slender points, and branching less abruptly. The roots of 

 the onion, hyacinth, and other bulbs, are good examples of the 

 simple unbranched form ; the roots of the grasses are much 

 branched ; but the degree of ramification of the branching roots 

 of Monocotyledons like that of Dicotyledons is apparently de- 

 pendent very much on external conditions. 



Perennial Monocotyledons, dying down annually, which do 

 not provide for their endurance by enlarged stem-structures 

 (buds), such as bulbs and corms, produce tuberous root-structures. 

 In our native orchids one adventitious root each year becomes a 

 tuberous root, to provide for the next season's growth ; and before 



