PERIODICITY IN DEVELOPMENT. II 357 



directly without any resting period (e. g. Fuciis). If follows that sexual fusion 

 is not the cause of the need for a resting period ; indeed it has been observed 

 that the gametes of Ulothrix under certain circumstances (Klebs, 1896, 321) 

 may develop into resting spores without any fusion, and the same phenomenon 

 has been for a long time known to occur in many species of Oedogoniiim (com- 

 pare HiRN, 1900, p. 39). If gametes are able to develop into resting spores with- 

 out any fusion, then conjugation must have some special significance of its 

 own, into which we shall go later on. 



The examples quoted are sufficient to render clear the historical lines of 

 investigation on this subject. Fungi are, however, much better adapted for 

 studies of this kind than Algae, because we are more thoroughly acquainted 

 with their conditions of life, and because these conditions are for the most part 

 not so restricted as in the case of Algae. An example of this has been already 

 given in Basidiobolus ranarum, but without referring further to this case we 

 may rather turn to the general conclusions which Klebs (1900a) has arrived at 

 from his researches on Fungi. In that group, far better than in Algae, we 

 see that reproduction is not an essential result of vigorous growth, as one would 

 be inclined to assume from the regularity with which reproduction follows such 

 growth. As a matter of fact, growth may proceed to an unlimited extent with- 

 out reproduction, provided the necessary external conditions are fulfilled. In 

 addition to favourable general conditions the most important is continuously 

 good nutrition, e. g. in cultures, a frequent renewal of the nutrient solution before 

 its nutritive value has appreciably diminished or injurious metabolic products 

 have accumulated. If, from time to time, a fragment of the mycelium of 

 Saprolegnia mixta be taken from a good culture of the fungus and transferred 

 to a fresh nutritive solution, it may be kept growing vegetatively for two and 

 a half years without reproducing. ' This phenomenon is illustrated far more 

 effectively by the case of yeast, which has, for hundreds of years, persisted in 

 the vegetative condition, and which can be induced to form spores only under 

 very special conditions.' Similar experiments have been carried out on a large 

 number of lower organisms, Myxomycetes, Bacteria, and Fungi, with the same 

 results ; it is more important to note that at least some higher types may be 

 similarly treated, e. g. Coprinus ephemerus, which may be cultivated for months 

 as a sterile mycelium without the formation of any pHeus. Experiments cannot 

 be carried out so successfully on Algae because it is often the case that the most 

 insignificant alterations in the conditions induce spore formation ; it may be 

 noted, however, that Vaucheria geminata will remain sterile in flowing water for 

 as long a period as may be desired. The factor which most commonly induces 

 the appearance of reproductive organs is an alteration in nutrition, especially 

 a reduction in the absorption of food. In the case of certain Fungi which form 

 fruits only in the air, transpiration is also a determining factor ; a saturated 

 atmosphere interrupts the formation of spores, but a reduction in the amount 

 of moisture induces the formation of spores in abundance. Light also plays a 

 part in a few cases, as in certain species of Coprinus and in Pilobolus microsporus. 

 It is only when light is permitted to play on the terminations of the conidio- 

 phores of Pilobolus that conidia are formed. Illumination for a few minutes is 

 often sufficient ; if it be not illuminated the conidiophores grow on just like 

 ordinary vegetative cells, so long as nutritive materials can reach them. 



The withdrawal of nourishment acts in the first instance as a stimulus to 

 the formation of reproductive organs, whOe, at the same time, it retards vegeta- 

 tive growth. A contrast would thus appear to exist between reproduction and 

 growth pure and simple, but it must not be assumed that reproduction begins 

 when nutritive conditions are unfavourable and growth only when they are 

 favourable. On the contrary, it appears that reproduction is all the more 

 vigorous the better the vegetative parts are nourished. It would thus appear 

 that growth is an essential precedent of reproduction. It is so, however, only in 



