238 COMPENDIUM OF GENERAL BOTANY. 



tase) must be present in order that the reserve materials may be 

 dissolved, since they cannot be utilized in the processes of nutrition 

 while in the solid state. 



In regard to the " annual vegetative period," we soon recog- 

 nize that this phenomenon needs further elucidation (PFEFFER).' 

 According to my opinion, the investigation of the properties and 

 peculiarities of organisms reveal relations and adaptations which 

 we can recognize as such without being able to explain them. Let 

 us consider a few examples. 1. We can see that the annual peri- 

 odicity is not peculiar to all plants ; the regularly recurring period 

 of rest seems to be a " facultative ' ' property of many plants 

 which manifests itself when desirable. 2. We can also see that 

 definite plant-forms are adapted to definite external relations. 



Ad 1. Many tropical plants while in their natural home 

 develop leaves and flowers during the entire year. If, however, 

 an annual dry period sets in, as is very common in tropical regions, 

 we notice that such plants undergo a periodic rest, corresponding 

 to the dry seasons. 



Ad 2. Climatic adaptability will have reached its limit when 

 palms can survive in our climate without artificial protection. Our 

 native oak and beech can exist in Madeira, but will shed their leaves 

 in spite of the moist, mild climate. The cherry is evergreen in 

 Ceylon, but does not develop fruit (DE CANDOLLE). According to 

 HUMBOLDT, the grape of Venezuela bears leaves and fruit during 

 the entire year. HARNIER noticed the same thing in the grapes 

 of central Africa (Khartoum). 



Y. THE GENERAL PHYSIOLOGY OF REPRODUCTION. 



In this chapter the statements are based essentially upon the 

 investigations of NAGELI and in part upon those of SACHS, unless 

 other citations are given. 



A. AGENTS IN FERTILIZATION. CROSS-POLLINATION. SELF- 

 POLLINATION. 



PFEFFER'S recent investigations have revealed the cause which 

 induces the free-swimming spermatozoids of cryptogams to move 



Pflanzenphysiologie, II, 106. 



