REPRODUCTION. 411 



It is in the animal kingdom only that we meet with in- 

 stances of this spontaneous division of an organic being into 

 parts, where each reproduces an individual of the same spe- 

 cies. All plants, however, are capable of being multiplied 

 by artificial divisions of this kind; thus, a tree may be di- 

 vided longitudinally into a great number of portions, or 

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

 rately and supplied with nourishment, may continue to 

 grow, and may, in time, reproduce a tree similar in all re- 

 spects to the one from which it had originated. This inhe- 

 rent 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, ultimately, a com- 

 plete plant will be formed.* These facts, which are well 

 known to agriculturists, exhibit only the capal)ilities of ve- 

 getative power under circumstances which do not occur in 

 the natural course of things, but have been the effect of hu- 

 man interference. 



Reproductive powers of a similar kind are exhibited very 

 extensively in the lower departments of the animal king- 

 dom. The Hydra, or fresh water polype, is capable of in- 

 definite multiplication by simple division: thus, if it be cut 

 asunder transversely, the part containing the head soon sup- 

 plies itself with a tail; and the detached tail soon shoots forth 

 a new head, with a new set of tentacula. If any of the ten- 

 tacula, or any portion of one of them, be cut off, the mutila- 



• Among the conditions necessary for these evolutions of organs .ire, first, 

 the previous accumulation of a store of nourishment in the detached frag- 

 ment, adequate to supply the growth of the new parts; and, secondly, the 

 presence of a sufficient quantity of circulating sap, as a vehicle fbr the trans- 

 mission of that nourishment. It has been found that when these conditions 

 are present, even the leaf of an orange tree, when planted in a favourable 

 soil, sends down roots, and is capable of giving origin to an entire tree. Ac- 

 cording to the observations of Mirandola, the leaf of the BryophyUum, when 

 simply laid on moist groimd, strikes out roots, which quickly penetrate into 

 the soil. (De CandoUe, Physiologic Vcgctale, ii. 677.) The leaves of the, 

 monocotyledonous plants often present the same phenomenon. 



