772 REMARKABLE I'HOl'ERTIES OF ROOTS. 



with erect stems and tliick stoin-nodea, e.g. the various species of Galeopais or 

 Polygonum, are extended flat on the ground from some accidental cause, only 

 a part of the stem rises up again after a time by a right-angled bend at one 

 of the nodes, the part next the fi'ee apex rising up, while the part nearest the 

 attachment remains prostrate on the ground. Contact with the soil acts as a 

 stimulus to the formation of roots on this latter portion, and they are produced 

 abundantly near the node from the knee-shaped bent portion, and penetrate 

 into the ground, functioning as absorbent and fixing organs. These slirubby 

 plants would not have developed any roots on their stem-nodes had they not met 

 with the accident and so been stretched on the ground. 



Cut branches of willow placed in water, wet sand, moistened soil, or moss, 

 develop roots in about a week at the place where they are in contact with the 

 water or damp objects mentioned; roots which are equally useful either as absorbent 

 or fixing organs. If the brandies had not been cut ofl' or treated in this way, no 

 roots would have been formed on them. These willow branches may be taken 

 as a type of the shoots of a great number of plants which all readily develop roots 

 from the stem when placed in damp surroundings. The propagation of plants 

 by cuttings, so often performed by gardeners, depends upon the fact that when 

 branches are cut ofi' from a plant and placed in damp sand they "strike root" 

 in it, i.e. they send out roots from the part of the stem situated in the sandy soil. 

 Contact with damp earth operates as an incitement to the formation of roots in the 

 aerial, cord-like roots of the ai-oids figured on p. 365, just as in these cuttings. The 

 aerial roots descending from the sterns of these aroids do not develop absorbent, 

 lateral i-oots until they reach the soil; but they have scarcely come into contact 

 with it when numbers of lateral roots arise which penetrate into the ground where 

 they can suck up fluid nourishment. In the root- forming leaves of species of 

 pepper, of begonias, and of the cuckoo flower, contact with damp soil stimulates 

 the production of roots — in places, too, where no roots would have been formed 

 without this contact. If a pepper or begonia leaf is cut in pieces, and each piece 

 laid on damp sand and so pressed down that the veins projecting from the lower 

 side are embedded in the sand, roots will grow out of the parenchyma adjoining the 

 veins, and turn downwards, while above they develop a tissue-body which turns 

 upwards and becomes a leafy shoot, being provided with food by the roots. Long 

 roots arise from the cellular tissue at the base of the stalk of rank ivy-leaves 

 placed in wet sand or in water, which is never known to happen when the ivy- 

 leavos are growing freely in the air. We must not omit to mention here the roots 

 of parasitic plants which attach themselves to the living tissue of other plants as 

 so-called haustoria; these only arise in the parts of the parasite which come directly 

 into contact with the succulent I'oots of the living host-plants. 



The benefit which plants derive from the formation of tiiese roots is easily 

 perceived. In the stems of the prostrated shrubs the conduction of fluid food from 

 the ground is, no doubt, restricted and imperilled, and therefore it is important that 

 the part of the shoot again rising from the ground should be provided with special 



