GERMINATION OF SEEDS 627 



they are brought into contact with any moist substratum or even simply ex- 

 posed to a saturated atmosphere, provided other environmental conditions are 

 also suitable. 



The initial step in germination is the imbibition of water by the various 

 tissues within the seed. This generally results in an increase in its volume. 

 The increase in the hydration of the seed coats usually causes a pronounced 

 increase in their permeability to oxygen and carbon dioxide, which is very 

 low in the dry seed coats. The swelling of the seed often ruptures the seed 

 coat, but in some species this does not occur until the emergence of the primary 

 root. 



With an increase in the hydration of the cells, enzymes become activated 

 and zymogens are converted into enzymes. In seeds possessing an endosperm, 

 enzymes apparently move into that tissue from the embrj'o as described in 

 Chap. XXVII. Stored foods, whether they occur in the endosperm or coty- 

 ledons, are digested and the soluble products of the digestion process are trans- 

 located towards the growing points of the embryo. If chemical analyses 

 are made of samples of seeds at successive stages during their germination 

 it is found that the quantity of starches, oils, and proteins in the seed de- 

 creases markedly (Table 37). A large proportion of the fats present are 

 usually converted, after digestion, into soluble carbohydrates. The soluble 

 carbohydrates are not present during the later stages of germination in amounts 

 quantitatively equivalent to the starch or other storage carbohydrates digested 

 during the process, indicating that a large proportion of these compounds is 

 consumed in respiration or assimilated in the construction of the carbohydrate 

 constituents of cell walls. In oily seeds the soluble carbohydrates utilized in 

 respiration result largely from chemical transformations of the products of the 

 digestion of fats. Digested proteins are usually represented in the seeds by 

 quantitatively equivalent amounts of amino acids, asparagine, etc. This in- 

 dicates that proteins are not consumed in respiration but are utilized in the 

 synthesis of the organic nitrogen compounds of the growing embryo. 



Insofar as the actual mechanics of seed germination are concerned two 

 principal groups of seeds may be recognized : ( i ) those in which the cotyledons 

 emerge from the seed and (2) those in which the cotyledons remain per- 

 manently within the seed. Most seeds of dicots and seeds of some monocots 

 such as onion belong to the first group while the seeds of grasses and of 

 some dicots such as peas and oaks belong in the second. 



I. Seeds in Which the Cotyledons Emerge. — The sequence of events that 

 takes place during the germination of the seed of the lima bean {Phaseolus 

 lunatus) will be described as a type example of this group (Fig. 138). 

 Germination is initiated by a marked swelling of the seed which usually rup- 



