GROWTH, IRRITABILITY AND MOVEMENT 141 



varies very considerably. Seeds of Willow (Salixs pp.) lose it a few 

 days after liberation from the parent plant ; those of a number of 

 other native trees lose their viability a few months after shedding. 

 But records are available of the long survival by seeds of periods of 

 enforced dormancy. Thus those of a leguminous species, taken from 

 a dried herbarium specimen, were still capable of germination though 

 known to be over 150 years old. Examples are also known of seeds 

 having lain dormant buried deep in the soil for very long periods. 

 When grassland, which has at some previous period been arable, is 

 again ploughed up, a crop of characteristic weed-plants of arable 

 land may be obtained in the first season. There is evidence that 

 the seeds producing these plants have lain dormant at low levels in 

 the soil for periods up to fifty years. It is possible that the dormancy 

 here is due to the high concentration of carbon dioxide present at 

 low levels in the soil. This will arrest their respiration, and hence 

 their growth (see p. 334). 



2. The Vegetative Phase of Growth. 



(a) General Features. 



Germination having been effected, the young plant enters upon 

 a period of growth in which the plant body characteristic of the 

 species is built up. The particular form assumed by a plant is the 

 result of the interaction of external conditions of growth with the 

 internal hereditary factors (Chapter XXXV.). 



Growth is initiated in the meristematic regions of the plant, where 

 new cells are formed. Meristems are found at the apices of shoots 

 and roots, and also in the form of cambium, often present in older 

 parts of the plant. In a meristematic cell new protoplasm is elaborated, 

 and this is usually accompanied by an increase in the size of the cell. 

 In due course division of the nucleus and protoplasm occurs, and with 

 the laying down of a new wall two daughter cells are formed. The 

 meristematic regions are characterised by the constant repetition of 

 these events. The influences that induce the continued activity of 

 the meristems are but little understood : there is, however, evidence 

 that cambial activity in the stem is induced by an activating 

 substance diffusing from apical regions (p. 1 50). The formation of 

 new protoplasm and of cell-walls in the meristematic regions neces- 

 sitates a supply of organic materials such as amino-acids, amides 

 and sugars. These are transferred to the growing points from the 

 leaves, where they are originally manufactured ; or from storage 



