REJIARKAIU.E rROPEUTIES OF ROOTS. 771 



l>y their growth in tliickness. In consequence of this the soil below the cylin- 

 drical, horizontal root becomes compressed, but that above it is raised and burst 

 open. The thiciv, woody root gradually becomes visible on the surface, and is 

 entirely stripped of earth on the upper side. The axis of the horizontal root 

 never again assumes the position of earlier years; tlicn the roots were only a 

 few millimetres thick, but now they have attained to a diameter of 20-30 centi- 

 metres, and the root-axis has been shifted upwards through almost half its 

 diameter, i.e. through 10-15 centimetres. The erect trunk, which is tirmly 

 united to the horizontal i-oots in the way just described, is, of course, raised up 

 to the same extent. In this manner may be explained the peculiar appearance 

 so frequently to be seen in our pine and oak forests — the appearance of huge 

 tree-trunks with thick woody roots springing from their base which are quite 

 devoid of earth on their upper sides, and run, half underground, in snake-like 

 coils into the forest ground. 



The elevation of stems by means of roots is more striking in tropical man- 

 groves even than in our native trees. After the seedling has fallen from the tree 

 and bored its way into the mud, protuberances arise on the circumference of its 

 lower third wliich develop into roots directed obliquely downwards. Even in a 

 few months the buried plant is raised up a little above the mud by the lengthening 

 of these roots, so as now to look as if it were supported on stilts. 



It has been repeatedly mentioned that the primary roots of the embryo 

 originate from places on the hypocotyl which have been determined beforehand. 

 So also does the origin of roots on many rhizomes, runners, and on climbing 

 stems seem to be precisely determined, and to be quite independent of external 

 iuHuences. For example, the primary root of mustard and numerous other 

 plants is developed under all circumstances from one pole of the hypocotyl. 

 The runners of strawberry plants and of the Creeping Crowfoot (Fragaria vesca 

 and Ranunculus repens) develop, without any external stimulus, a group of from 

 two to five root-protuberances on the stem-nodes, and the bramble branches, 

 described above, curve like arches to the ground, forming several root-prominences 

 at definite spots near the apex before they have reached it, in order that they may 

 take root in the soil. In many epiphytic aroids and orchids the places of origin 

 of the roots are arranged as symmetrically round the stem as are those of leaves, 

 and many other examples might be cited from which it follows that the position 

 of part of the roots is definitely fixed beforehand, being based upon the specific 

 constitution of the protoplasm of the species in question. But as well as the 

 roots developing in the manner indicated in definite positions, others are formed 

 which require for their development a special stimulus from outside, whose 

 place of origin is not determined beforehand, but is first fixed by some external 

 agent. To this category belong the roots arising from the nodes of shrubs which 

 have been battered down on the ground, and from stems coming in contact 

 with damp objects, as well as those which proceed from foliage-leaves, and, 

 finally, the wart-like roots of parasites known as haustoria. When shrubs 



