SECT. IT PHYSIOLOGY 281 



which have fallen into -water, and their thallus first grows through the body of 

 the insect. After a time however it grows out and forms a radiating growth 

 around the insect. The end of each of the radiating hyphae becomes as a rule cut off 

 by a septum, and its contents divide up into numerous swarm-spores ; these 

 emerge, move about, and finally germinate to give rise in another place to a new 

 individual of Saprolegnia. Later eggs and sperm-cells are formed on the older 

 plant and, at least in some species, the former only develop after being fertilised. 

 With the production of fertilised eggs tbe activity of the Saprolegnia plant tends 

 to cease ; it gradually perishes. 



G. Klebs has shown that it is possible to completely change this course of 

 development of Saprolegnia ; Klebs has succeeded in directing tbe development 

 in the following ways among others : — 



1. The mycelium can continue for tbe whole year to grow vegetatively when 

 supplied continually with fresh and suitable nutritive material. 



2. Such a well-nourished mycelium on being transferred to pure water proceeds 

 completely and at once to form sporangia. 



3. In solutions of leucin (O'l %) and haemoglobin (O'l %) at first a strong 

 growth develops and then sexual organs are formed. Swarm-spores are not 

 formed ; they appear, however, after the sexual organs, when a more dilute 

 solution (0"01 %) of haemoglobin is employed. 



It is thus clear that quite definite conditions exist for vegetative growth, 

 others for the formation of sexual organs, and yet others for the appearance of 

 asexual reproduction. 



In Phanerogams the question of the causes of the development of flowers is of 

 special interest. Observations in nature and experimental work show that in this 

 case also sexual reproduction is not absolutely essential to the maintenance of the 

 species, and that the formation of flowers only takes place under quite definite 

 conditions. The results which Klebs obtained with Sempervivum Funkii can be 

 sumarised thus : — 



1. AVith active carbon-assimilation in bright light and rapid absorption of 

 water and nutrient salts, the plant continues to grow purely vegetatively. 



2. With active carbon-assimilation in bright light, but with limitation of the 

 absorption of water and salts, the development of flowers takes place. 



3. With a moderate absorption of water and nutrient salts it depends on the 

 intensity of the illumination whether vegetative growth or the production of 

 flowers takes place. With weaker intensity of light, and when blue light is used, 

 only growth takes place ; with stronger illumination or with red light flowering 

 occurs. 



There are thus a number of observations and experiments which enable us to 

 see that light, temperature, and supply of nutrient salts are of significance in 

 determining flower formation. 



The importance of light in the formation of flowers is shown by the well-known 

 fact that the Ivy only flowers when growing in a well illuminated situation and 

 not in the shade of woods, although it grows well in the latter habitat. Voechting's 

 experiments on Mimulus Tilingii gave the same result. At a certain low 

 intensity of light, which is quite adequate for vegetative growth, this plant 

 produces no flowers. This effect of diminished light becomes evident even when 

 the floral shoots have been formed ; the rudiments of the flowers in the axils 

 of the bracts are arrested in their development, and axillary buds which would 

 otherwise remain dormant grow out as leafy shoots. There is a return to the 

 vegetative condition. Klebs has made corresponding experiments with Vcrnnica 



