4i8 



PHYSIOLOGY 



material. Plants, therefore, do not in general have a definite stage of 

 maturity, and a corresponding form. They do have, however, periods 

 characterized by growth, including the formation of new organs and their 

 development. These periods occur once, being limited to a single season 

 or less, as in the case of annuals; or twice, as in biennials; or they are 

 repeated, season after season, as in perennials. This periodicity is less 

 marked in equable tropical climates, but is rarely, if ever, entirely absent. 

 Phases. — If the history of any limited portion of a plant be followed 

 (and the more limited the better, even to a single cell), it can be observed 

 to pass through a development in which may be recognized three phases. 

 The first phase may be called the formative phase; the second, the phase 

 of enlargement; and the third the phase of maturation. These phases 

 are characterized clearly enough by certain peculiarities of structure 

 and behavior, but they are not sharply delimited. On the contrary, 

 the first passes by imperceptible gradations into the second, and the 

 second into the third; then growth finally ceases, unless some unusual 

 stimulus brings the cells again into an active state. 



Formative phase. — The formative phase is the earliest. Every plant 

 begins its existence as a single cell, and even when this one has increased 



to many, they usually remain 

 practically alike. The embryo in 

 seed plants, at the time when it 

 resumes its interrupted growth, 

 usually consists of cells all in 

 the formative stage. They are 

 characterized by a relatively large 

 nucleus, abundant cytoplasm with 

 only minute vacuoles, and thin 

 walls. In this phase the frequent 

 division of the cells is a feature, 

 and in consequence of the more 

 rapid production of new cells by division at certain points, the primordia 

 of new organs appear (fig. 666). Some of the simpler plants never get 

 beyond this phase, except as to their reproductive organs. Even in the 

 larger plants, some of the cells permanently retain these characters, and 

 so constitute formative centers or growing points; but far the greater 

 number pass gradually into the second phase and the third, assuming 

 quite a different aspect and behavior. In particular, the power of 

 division is given up. 



Fig. 666. — Growing point of Hippuris. 

 -After De Bary. 



