DIV. ii PHYSIOLOGY 311 



It will be seen in the Special Part how various are the arrange- 

 ments to ensure the formation of reproductive bodies in the vegetable 

 kingdom. The division of the latter into classes, orders, etc., is 

 mainly based on this variety. Two types of reproduction can, how- 

 ever, be readily recognised throughout. These are vegetative and 

 sexual reproduction, and may also be termed monogenic and digenic 

 respectively, since only one organism is concerned in vegetative and 

 two in sexual reproduction. 



The organs which serve for reproduction have been treated in 

 the section on Morphology. In this place the conditions and the 

 significance of the phenomena have to be considered and the properties 

 of the offspring discussed. 



1. The Conditions of Reproduction ( 74 ) 



In nature reproduction appears to follow vegetative growth with 

 some degree of necessity. It commences as a rule when the vegetative 

 growth is slackening and the plant has attained a certain age. It 

 can, however, be shown that this succession is not obligatory, and that 

 the natural course of development is determined by quite definite 

 conditions, and can be greatly modified by other influences. 



Thus the question arises, under what conditions does vegetative 

 growth and under what conditions the formation of reproductive 

 organs respectively take place ? Since these problems have as yet 

 been relatively little studied, it is not easy to give a general answer 

 to this question. We must, therefore, confine ourselves to making clear 

 the essential facts by means of some examples. 



Lower Plants. The fungi belonging to the genus Sapi'okgnia 

 have a non-septate, branched mycelium without chlorophyll. They 

 occur commonly in nature on dead insects 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 the 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 direct- 

 ing the development in the following ways among others : 



1. The mycelium can continue for the whole year to grow vege- 

 tatively when supplied continually with fresh and suitable nutritive 

 material. 



