CHARACTER AND RATE OP GROWTH IN STEMS 217 



toward the stem's base where the grand period is over, there are 

 internodes in various stages of the grand period. Due to the 

 overlapping of the grand periods of the different internodes, the 

 elongation of the stem as a whole is quite uniform. In roots 

 the grand period is passed through very quickly and is evident 

 only near the tip. 



The rate of growth also depends much upon the kind of plant 

 and upon moisture, temperature, and light. While some plants, 

 like Corn and Giant Ragweeds, grow to a height of six feet or more 

 in three or four months, the seedlings of some Pines and Oaks 

 grow only a few inches during an entire season. Some vines like 

 the Hop plant may grow a stem more than twenty-five feet in 

 length in one growing season. Most weeds grow more rapidly 

 than cultivated plants and, if let alone, soon exceed them and cut 

 off the light. Measurements have shown that some kinds of 

 Beans and Peas can elongate about two inches and Wheat about 

 four inches in forty-eight hours. In perennials, such as trees, 

 growth is very rapid in the spring, after which it slows down dur- 

 ing the remainder of the season. 



The moisture of the soil and air is an important factor in growth. 

 It is common knowledge that plants are checked in growth when 

 the ground becomes dry. The moisture of the air, although not 

 of use to the plant in the same way that the soil moisture is, 

 checks the evaporation from the plant and thereby influences 

 growth. When the atmosphere is full of water, as on "muggy" 

 days, there is not much evaporation and the cells easily retain the 

 high turgor pressure upon which rapid growth depends. It is 

 partly due to the greater humidity at night that many plants 

 grow faster then than in the day time. That the cells of plants 

 are often more turgid at night than in the day time is shown by the 

 fact that soft stems, like those of Corn and Sorghum, are more 

 flexible and not so easily broken off in the latter part of the day as 

 they are at night or in the morning. For this reason, the after- 

 noon, when the cells are least turgid, is the best time to lay-by 

 Corn. The function of water in enabUng cells to stretch is an 

 important one, for enlargement consists in stretching the proto- 

 plasm and cell walls without much increase at first in dry weight. 

 Thus the dry weight of an internode of a stem is about the same 

 at the end as at the beginning of the grand period, although the 

 size may increase many times. In fact seedlings, before they 



