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. 
F:G. 666. Growing point of Hippuris. 
After DE BARY. 
