358 METAMORPHOSIS 



so far as growth is associated with vigorous assimilation, for it is obvious that 

 the construction of reproductive organs necessitates the previous accumulation 

 of a certain amount of nutritive materials, and these must be all the more 

 abundant the more complex the reproductive organs are. Thus simple repro- 

 ductive bodies may arise from other organs of propagation without growth, 

 provided these have a sufficient supply of constructive materials. The best 

 known example of this is seen in the swarmspores of Oedogonium which after 

 coming to rest may become swarmspores once more. KLEBS hasdrawn attention 

 to similar phenomena in many Fungi. 



A more important difference between growth and reproduction lies in this, 

 that general vital conditions are not so restricted in relation to the former as the 

 latter. For example, growth may still go on at a very low or a very high 

 temperature, while formation of reproductive organs ceases. Oxygen, light, 

 concentration of the nutritive solution, the quality of the nutrient, all have 

 minima and maxima which are much closer together in the case of reproduc- 

 tion than in the case of growth. It must not be assumed, however, that the 

 optimum for the one process is also that for the other. 



Observations similar to those which have been made on Algae have also 

 been carried out on Fungi, both as regards the occurrence of the reproductive 

 organs in general and of the various types of these in particular. Fungi far 

 excel Algae in the variety of their methods of spore formation. Interesting 

 as are the results achieved by KLEBS and his pupils on this subject we must not 

 attempt to describe them here, for their complex nature renders it impossible 

 to summarize them in a few sentences, and space will not permit of more. 



If we turn now to the higher plants, at first sight we appear to meet with 

 an essentially different state of affairs. An oak, for example, has, like a higher 

 animal, only one type of reproduction, i. e. the formation of an embryo con- 

 tained in a seed. This embryo is the product of a sexual act, which takes place 

 when the plant has reached a certain age, and is seemingly induced by internal 

 causes. Closer study, however, reveals something entirely different. In order to 

 understand the phenomena we must study reproduction in the fern, for a 

 knowledge of the process in the latter is essential to a true conception of what 

 follows. The fern has the same complicated structure as the flowering plant, 

 consisting as it does of a root-system and a leafy shoot. On the under sides 

 of the leaves arise, in definite ways characteristic of the group, asexual spores 

 enclosed in special receptacles, the sporangia. The spores generally pass through 

 a resting period and germinate under favourable conditions. The plant which 

 arises from the spore resembles, however, a liverwort rather than a fern. It con- 

 sists of a cellular expansion a few millimetres in diameter, increasing by means 

 of an apical cell, and attached to the soil by rhizoids. This second generation 

 derived from the spore is termed the prothallus, and is incapable, as a rule, of 

 developing by simple growth into the first stage. When its reproductive period 

 comes on it develops sexual organs. Archegonia, corresponding to the oogonia 

 of Oedogonium, appear (Fig. 108), and each of these contains, as its essential 

 constituent, an ovum. The ovum is fertilized by a motile sperm, produced in 

 a special receptacle, an antheridium (Fig. 107), and from the product of the 

 fusion of these two cells there arises a new fern plant, which is at first dependent 

 on the prothallus, but later on, after the death of the prothallus, is capable of 

 independent existence. In the natural course of events the two generations 

 in the fern follow each other regularly, the asexual generation or sporophyte 

 being succeeded by the sexual gametophyte, and that in turn by the sporo- 

 phyte ; in other words, we have here a case of alternation of generations. Any 

 attempts to induce a spore to form a sporophyte or an oosperm to form a 

 gametophyte by external agencies have been as yet entirely unsuccessful. In 

 this respect the fern shows a marked difference from the Algae and Fungi. 



This distinction cannot, however, be fundamental, for we know of several 



