PERIODICITY IN DEVELOPMENT. II 353 



organs of reproduction. What is the factor which all at once determines this 

 change in the organism, viz. the arrest of the formation of vegetative organs and 

 initiation of the development of reproductive-leaves ? This is the problem in 

 developmental physiology which forms the subject of consideration in the 

 present lecture. First of all, let us be sure that we understand what ' repro- 

 duction ' really means. 



The current conception of reproduction has been derived from the con- 

 sideration of the phenomena as exemplified by the higher plants, and especially 

 by animals. In animals the individual is said to reproduce when it gives rise to 

 individuals like itself. The conception of the ' individual ' as understood by the 

 zoologist cannot be applied to the plant world, for in many growth-phenomena it 

 is extremely difficult to say whether we are dealing with continued growth of 

 the parent or with the formation of a new organism. No one would dream of 

 applying the term ' reproduction ' to the formation of a lateral branch on the 

 stem of a willow, for the young branch is exactly like the parent, and obviously 

 is merely adding to the extent of the original tree ; but suppose this very branch 

 were torn off by a gale of wind, and suppose it rooted itself and developed into 

 a sapling, we have to decide now whether this is a reproductive phenomenon or 

 not. The isolation of twigs, which may in this illustration be purely accidental, 

 occurs in many plants quite definitely and regularly. Thus in many rhizomes 

 the branches become isolated by decay of the older parts behind, and these 

 isolated branches give rise to new and independent plants. In such cases 

 what must one call this process ? At the beginning it is obviously merely 

 growth and branching, later on it becomes reproduction. Where must we 

 draw the line of demarcation ? The difficulties do not decrease when simple 

 forms such as Algae are considered. Spirogym, for instance, consists of cylin- 

 drical cells united into cell filaments. Each cell is completely independent ; it 

 grows and divides, and thus the filament increases in size. Under definite 

 external conditions, however (BENECKE, 1898), .the filaments break down into 

 their constituent cells, each of which has the power of developing into a new 

 filament. Now if we regard the cell- filament as a unity, reproduction consists 

 in the segmentation of the thread into individual cells, but if each cell be an 

 individual we have no reproductive process represented here at all. 



With these examples before us we are justified in saying that often in 

 nature no hard and fast line can be drawn between vegetative growth and re- 

 production. It will serve our present purpose better, however, if we institute 

 an arbitrary limit and regard the process of reproduction as always involving 

 the formation on the part of the plant of special organs whose duty it is to give 

 origin to new individuals of the same species. Such organs must of necessity 

 be capable of separating from the parent plant ; they must contain a certain 

 amount of protoplasm capable of development and with possibilities of access 

 to reserves of food-material required to render further development possible. 

 This protoplasm may be in the form of a single cell or a cell-complex, whether 

 occurring in higher or in lower plants, and the cell-mass may take a form as 

 complex even as a growing point or bud. Let us commence with a simple 

 example. 



Ulothrix zonata is an alga whose cylindrical cells are combined to form a 

 simple unbranched filament, but it differs from the majority of the forms of Spiro- 

 gyra to which we have previously referred in being composed of cells which are not 

 all alike ; one of them, the basal cell, exhibits the peculiarity of acting as a fixing 

 organ (Fig. 105, A, r). Reproduction in the alga takes place as follows. The 

 contents of a cell subdivides into two or more cells (B), each of which contains, 

 in addition to protoplasm, a chloroplast and a nucleus. Further, each is provided 

 at its colourless anterior end with four delicate protoplasmic filaments or cilia, 

 by whose vibrations the naked cell moves through the water (C) after its escape 

 from the mother-cell through a cleft in the membrane. It has been customary 

 OST A a 



