662 LECTURE XXIII. 



We will now, in briefly criticising these various theories of 

 reproduction, endeavour to ascertain which of them most 

 naturally explains the facts before us. 



There can be no doubt the idea which underlies Naegeli's 

 theory of reproduction, the idea, namely, that the protoplasm 

 of each organism constitutes one complete whole, is the only 

 reasonable foundation on which to frame a theory of repro- 

 duction. The protoplasm of every plant presents a certain 

 definite form, or a certain external segmentation, and if a 

 portion of that protoplasm be isolated it will, if it can grow at 

 all, grow in such a way as to reproduce the form and seg- 

 mentation of the mass of protoplasm of which it once formed 

 part. This may be illustrated by an analogy borrowed from 

 Pfliiger. If a small imperfect crystal of any salt be placed in 

 a saturated solution of that salt, it will first repair its crystal- 

 line form, and then increase in size. An exact parallel is 

 afforded by a cutting. The cutting, let us say, formed part 

 of a plant the protoplasm of which was segmented into stem, 

 leaf, and root. The segmentation of the cutting is incom- 

 plete ; it has no roots ; it is comparable to the imperfect 

 crystal. On being planted, however, the cutting completes its 

 segmentation by producing roots, just as the imperfect crystal 

 in the saturated solution completed its crystalline form. 



It may be objected that this analogy does not hold good 

 in all cases. There is the fact, for instance, that when a stem 

 is cut across, a new stem is not developed at the cut surface. 

 On examination, however, it will be found that this objection 

 has no real weight. In the case of most plants, though it is 

 true that the original stem will not be replaced, and that the 

 surface of the wound will become covered by a layer of 

 callus, yet a new shoot is developed from one of the lateral 

 buds; so that after all the protoplasm of the plant is still 

 segmented into root and shoot. In the case of plants which 

 have no lateral buds, there is no possibility of repair, and so 

 the injury proves fatal. 



We may regard the development of an individual from a 

 spore in like manner. The protoplasm of a spore may be 

 compared to a minute complete crystal ; and just as such a 



