362 METAMORPHOSIS 



development of sexual organs appears to be a necessary consequent of the forma- 

 tion of spores physiological investigation must be limited to the interpretation 

 of the conditions of spore- and ;^oz£'g;'-production. 



As in certain ferns, so also in many Angiosperms, we meet with accessory 

 organs of reproduction, at least in the sporophyte stage, and when we speak of 

 these as asexual or vegetative reproductive methods we do not mean the mega- 

 and micro-spores (although these also are asexual in character), but those special 

 accessory organs which invariably consist of a growing point or bud, giving 

 rise to leaves and roots after these or the organs bearing them have separated 

 from the parent. Great variations occur in the point of origin of these buds, 

 and in the way in which the necessary reserves are stored in leaf, stem, and 

 root ; these, however, we need not discuss here. 



After this superficial sketch of the morphology of the reproductive organs 

 of the flowering plant we may pass to the problems requiring consideration from 

 a physiological standpoint, viz. what determines flower formation, what vege- 

 tative reproduction ; what relations exist between reproduction and vegetative 

 growth ; what is the special significance of reproduction, and more especially 

 of asexual and sexual reproduction. 



We are not nearly so well acquainted with these subjects in connexion 

 with higher plants as in the case of the lower, and the experimental investigation 

 of the former is much more difficult. The first question we have to answer here 

 is whether or not unlimited growth is possible without reproduction. At the first 

 glance the answer would appear to be in the negative, but further consideration 

 shows that that answer is incorrect. A tree may be several hundred years old 

 and still go on growing ; in the long run it dies, but we cannot say that death 

 was due to internal factors and that perpetual vegetative growth was impos- 

 sible. Death may be due rather to external factors, e. g. the ever-increasing 

 difficulty of conducting water to the lofty growing points. Klebs (1903) 

 attempts to account for the death of trees by assuming that certain decompo- 

 sition products migrate from the dead parts and gradually infect the living 

 organs. Our suggestion above might be made more general by saying 

 that when a certain size is exceeded nutritive disturbances (not merely inter- 

 ruptions in the water flow) generally ensue, which in the long run bring about 

 death. In many of the lower organisms also, e. g. in Hydrodictyon, con- 

 tinuous vegetative growth appears to be impossible. When the cells of this 

 alga come to exceed a certain size its central cells are badly supplied with food- 

 material and must in the long run die off unless the size of the body be reduced 

 as a result of reproduction. Internal factors are also known to be effective, 

 but they do not affect the parts of the tree which are of special interest to us. 

 The protoplasm of the growing points in an old tree has just as much power of 

 development as that of one a single year old. In proof of this view we point 

 to the numberless cultivated plants which are propagated by cuttings, and 

 which have been so propagated for hundreds of years. This is true, for example, 

 of the willow, poplar, sugar-cane, and many others; at the same time, some of 

 these plants in their later years are often subject to diseases which have been 

 considered to be a consequence of degeneration due to perpetual vegetative 

 propagation. It has, indeed, been maintained that the life of cuttings is limited 

 by that of the mother-plant from which they are taken. Mobius (1897) has, 

 as it appears to us, demonstrated beyond criticism that this view is incorrect, 

 and we may refer directly to the evidence he has advanced. It will be as well, 

 however, if we cite one or two examples in illustration of the position we hold, 

 viz. that vegetative growth may be unlimited. At the commencement of the 

 present lecture we referred to the case of rhizomes which grew on from year to 

 year, in the main axis or in lateral shoots. Since they are always forming new 

 roots acropetally all difficulties in absorption of water are removed. The 

 older parts, however, die off, and hence it is not possible in nature to deter- 



