20 



Introduction to Botany. 



The mangrove is a notable example of a plant whose 

 seeds habitually pass through the first stages of germina- 

 tion before falHng from the tree (see Fig. i). In most 

 cases, however, the seeds must go through a greater or 

 less period of rest before they are in a condition for ger- 

 mination ; and during this period of comparative inactivity, 

 ferments are possibly being formed which are necessary 



to render the starches, oils, and 



some forms of proteid soluble in 

 the cell sap. 



10. Conditions Necessary to Ger- 

 mination. — Until the internal con- 

 ditions are favorable through the 

 formation of ferments, etc., the 

 seed cannot germinate, but cer- 

 tain external conditions are also 

 necessary to germination. These 

 are the presence of oxygen, water, 

 and a certain degree of heat. 

 While some seeds have been 

 known to germinate at tempera- 

 tures quite close to the freezing point, most seeds germinate 

 best between i6° and 27° C. (60.8° and 80.6° F.). This sig- 

 nifies that until a certain amount of energy in the form 

 of heat is afforded to seeds from the outside, some of the 

 necessary processes attending growth cannot be initiated. 

 Neither can germination begin until the seed has absorbed 

 sufficient water to stretch its tissues and act as the solvent 

 for its reserve food materials. Some seeds are able to 

 absorb water until their tissues are stretched by a force 

 equal t6 about 200 pounds per square inch ; and it is this 

 stretching force which starts the increase in size of the 

 embryo plant. If oxygen is excluded from seeds, they will 



Fig. .i. 



Photomicrograph of Starch Grains 

 in a section of a grain of Indian 

 corn. Highly magnified. 



