REPRODUCTION. 525 



of which, at a certain period of its existence, tlie 

 ends, constituting the soft and contractile portion of 

 the animal, each having its separate mouth and 

 tentacula, detach themselves from the horny stem 

 to which they had previously adhered, and begin 

 their independent existence by swimming freely in 

 the water: in this state they have a great resem- 

 blance in form to small medusae. The stem which 

 has been thus abandoned, continues to live and 

 soon repairs its losses by the reproduction of new 

 polypes.* 



It is in the animal kingdom only that we meet 

 with instances of this spontaneous division of an 

 organic being into parts, where each reproduces an 

 individual of the same species. All plants, how- 

 ever, are capable of being multiplied by artificial 

 divisions of this kind : thus a tree may be divided 

 longitudinally into a great number of portions, or 

 slips, as they are called, any one of which, if planted 

 separately and supplied with nourishment, may 

 continue to grow, and may, in time, reproduce a 

 tree similar in all respects to the one from which it 

 originated. This inherent power of reproduction 

 exists even in smaller fragments of a plant; for, 

 when all circumstances are favourable, a stem will 

 shoot from the upper end of the fragment, and roots 

 will be sent forth from its lower end ; and ulti- 

 mately a complete plant will be formed. | These 



* Nordman, Comptes Rendus, ix. 704. 



t Among the conditions necessary for these evolutions of organs 

 are, first, the previous accumulation of a store of nourishment in the 

 detached fragment, adequate to supply the growth of the new parts; 

 and secondly, the presence of a sufticientquantity of circulating sap, 

 as a vehicle for the transmission of that nourishment. It has been 

 found that when ihese conditions are present, even the leaf of an 



