E. V. Wulff — 144 — Historical Plant Geography 



These habitats may sometimes be of exceptionally limited extent, 

 comprising, for instance, a single mountain or canyon. A certain kind 

 of mountain rock, e.g., chalk cliffs, may constitute the only refuge of 

 such relic plants. Within the limits of their restricted habitats these 

 species will continue to be propagated by seeds, if they have retained 

 the ability to produce them, or vegetatively, if they have lost this 

 ability. These "left-over" plants mix with the newcomers and form 

 one of the elements of the new flora of the given district. 



The question arises: Is it possible, by studying the biology of these 

 remnants of a former flora, to elucidate their past history and deter- 

 mine the changes that they have undergone in their habitat? To this 

 question we may reply affirmatively, since various factors in the life of 

 these plants provide definite clues to the knowledge desired. 



Periodicity in the Growth and Development of Plants: — That 

 vegetative processes, and also flowering and fruiting, succeed one an- 

 other in regular fashion is a well-known phenomenon, regarding the 

 causes of which there exists a rather extensive literature (see Drude, 

 1913). This alternation of vegetative and reproductive processes, this 

 periodicity in the passing through of the various phases of development, 

 is inherent to all plants, at least all higher plants, regardless of their 

 place of abode on the globe. Even in the tropics, despite the uni- 

 formity of climatic conditions, the vegetation is characterized by this 

 rhythm of development. Thus, Schimper (1898) pointed out that in a 

 tropical forest, notwithstanding the fact that climatic periodicity is 

 practically absent, there exists a periodicity in the life of the trees, 

 expressed in the falling of leaves at definite intervals and in an alter- 

 nation, even in the case of evergreen species, of periods of intensive 

 growth and periods of dormancy. 



We shall not go into the question (still a matter of dispute) as to 

 whether periodicity in the life of a plant is primary in character, i.e., 

 independent of external conditions, which may only direct it to one or 

 another side (Schimper, 1898; Volkens, 1912), or whether it is the 

 result of external conditions, since absolute invariabiUty of climatic 

 conditions during the entire year does not exist anywhere on the globe 

 (Klebs, 191 i). For our purposes a solution of this problem is not 

 vital. For us it is of importance, first of all, to establish the fact of 

 periodicity of phenomena in the plant kingdom and the existence of 

 interrelations between this periodicity and climatic conditions; and, 

 secondly, to establish the fact that the inherent periodicity of a plant is 

 in most cases hereditarily fixed and not readily subject to change with 

 a change in climatic conditions. 



In this connection of particular interest is an experiment made by 

 DiELS (191 8) with a number of different species of forest herbs. These 

 herbs were grown in a greenhouse under conditions of uniform tem- 

 perature (in contrast to the variable climate of their ordinary habitats). 

 The herbs could be classed in three groups, according to how their 

 dormant period was affected: In the first group (type— ^5/>erM/a) the 

 dormant period was completely eliminated; in the second (type— 

 LeMco/wm)- partially eliminated; and in the third {type— Polygonatum) 

 —fully preserved. These reactions clearly reflected the geographical 



