GERMINATION, GROWTH, TISSUE TENSION. 145 



exposed to suitable conditions, placing along with it other seeds of 

 the same species for comparison. 



Place some dry Beans or Peas in a dry, large test-tube, and an 

 equal number of soaked seeds in a test-tube half filled with water. 

 Cork both tubes and immerse them in a beaker of water kept at 

 60 C. for two hours on a bath other temperatures and periods of 

 exposure should be tried. Then soak the dry seeds in water, and 

 sow both lots, labelled, in your germination jars and boxes, and 

 expose the two batches to the same conditions. Find out in this 

 way how dry and soaked seeds differ in their ability to withstand 

 the effects of high temperatures. 



186. Effects of Cold on Seeds. We find that dry seeds can 

 withstand high temperatures which are fatal to soaked seeds. On 

 placing seeds among ice or a freezing mixture, we find that dry seeds 

 can also resist low temperatures that kill soaked seeds. Dry seeds 

 can germinate after being exposed for a long time to the most in- 

 tense cold that can be obtained, while soaked seeds are often killed 

 by exposure to the freezing temperature of water or a few degrees 

 below this. Repeat 185, but immerse the two tubes in a freezing 

 mixture, or place the two lots of seeds on ice instead of using hot 

 water. 



187. Is Air necessary for Germination? This is easily 

 tested either by depriving the seeds of air, or by confining them in 

 a series of closed vessels containing differeht volumes of air and 

 comparing the results. 



(a) Drop some seeds into a glass jar or wide-necked bottle, fill up 

 with water and cork tightly. As a control, put some soaked seeds 

 into a similar jar, leaving it open and adding a little water each 

 day to prevent the seeds from becoming dry, but not enough to 

 cover them. Ordinary tap-water contains dissolved air, but as a 

 rule seeds immersed in it, in a corked bottle, do not germinate ; to 

 make quite sure that no air reaches the seeds, the water should be 

 previously boiled to expel the dissolved air, and the cork sealed air- 

 tight with vaseline or plasticine. To hold the seeds down, fix them 

 into a spiral coil of wire, easily made by winding iron or brass wire 

 round a tube or a stick, 



(6) Take four .glass jars, all of the same size, and provided with 

 well-fitting corks. Fill these jars to different heights with moist 

 sand, marking each jar into five equal parts, and putting into the 

 first jar enough sand to reach the lowest mark ; into the second, 

 sand up to the next mark ; and so on. The fourth jar will thus 

 contain four times as much sand, and therefore only a quarter as 

 much air, as the first. Into each jar now place a dozen soaked seeds 

 (e.g. Cress, Wheat), cork tightly, and peal with plasticine and vase- 

 line. In which jar do the seeds germinate best? Do the results 

 P. B. 10 



