756 LIFE : OUTLINES OF GENERAL BIOLOGY 



by regeneration ; moreover, breakages during moulting are generally 

 made good. Yet since spiders are usually agile and elusive, it is not 

 surprising to find that loss, followed by regeneration, is not character- 

 istic of their order. But when we pass to the distantly related 

 arachnid order of "Harvestmen", or Opiliones, where the limbs are 

 elongated out of all proportion to the body, so that the risks of 

 breakage or seizure are obviously great, we find facile autotomy and 

 characteristic regenerative power. Many similar instances might 

 be given. 



Regeneration in Plants. — In the vegetable kingdom the 

 regenerative capacity is so widely distributed that it excites little 

 remark. Every gardener knows that he can easily propagate Begonias 

 by planting — or even laying — a leaf or a fragment of leaf on moist 

 sand or soil, where it speedily develops root-hairs on its under side. 

 These root-hairs normally arise in most species when the leaves 

 come in touch with the damp ground, but not in Begonia rex, where, 

 however, they can be experimentally induced. The growing of 

 root-hairs is the first step, and ensures a supply of water before the 

 excised piece of leaf can dry up. The second step is the development 

 of adventitious buds at the cut edge of the leaf ; and the number of 

 these can be increased by making several nicks on the chief veins; 

 so upon a single leaf a good many new plants can be grown. Similar 

 experimentation by skiUed propagators has been proving that what 

 is familiar, yet seeming exceptional, in the case of Begonias, can be 

 elicited in leaves of many different orders, in which it was never 

 known to occur in Nature, or in cultivation. 



As one would expect, the regenerative capacity is expressed in 

 varying degrees. Thus in some types, e.g. flowering plants, the excised 

 leaf or portion can develop root-hairs, but no buds, so that no actual 

 multiplication occurs. This is very limited regeneration, but at the 

 other extreme, as sometimes in the common cuckoo-flower, Cardamine 

 pratensis, buds may arise on growing and uninjured leaves. Very 

 interesting in its adaptiveness is the Mexican stonecrop, Sedum 

 Stahlii, in which the plump leaves are very loosely attached, and 

 easily jerked off by a slight knock, such as might come from a bird's 

 foot. Such a detached leaf develops adventitious buds which take 

 root. Yet in another species, S. album, the leaf has the same 

 power of forming adventitious buds, though their firm attachment 

 in this case robs the regenerative capacity of its usefulness for 

 discontinuous dispersion, though furthering extension in continuity 

 with the parent plant. 



In lower plants, such as most mosses, liverworts, and algae, there 

 is almost unlimited power of directly growing a whole from a part. 

 In higher plants, the first step is usually the formation of an adven- 

 titious bud, from leaf or shoot, and in a few cases even from 

 a root. 



