70 PHYSIOLOGY. 



others be found merely containing air : this air has 

 been analysed by Bischoff and Focke, the former stating 

 it to be rich in oxygen, and the other in carbonic acid ; 

 but, as Meyen remarks, it must vary at different periods 

 of the day, as in fact Bischoff experienced. In regard 

 to ducts, the period allotted to their conveyance of fluid 

 is probably greater than that of spiral vessels, as is also 

 the velocity and ease of its conduction ; and in the 

 proper ducts the transmission of the fully elaborated 

 juice, the true sap, evidently takes place. 



6. Intercellular substance, air cells, air passages, re- 

 ceptacles of secretion, etc., betray their office by their 

 name. 



7. We shall now speak of the various functions of the 

 plant in detail, commencing with those actions which 

 take place at what may be termed the birth of the new 

 being, and proceed gradually up to that period of its 

 life when it comsummates all it was in one respect in- 

 tended for, the production of other creatures like itself. 



GERMINATION. 



8. Under germination we are to consider the burst- 

 ing forth of the embryo of a seed, when placed under 

 certain and proper conditions, into a new and progres- 

 sively perfect plant ; the circumstances seen to accom- 

 pany it, and the conditions that must necessarily exist 

 for such processes to ensue. 



9. The essential conditions of germination are the 

 presence of oxygen, and a certain degree of tempera- 

 ture ; but in a general way we may say they are watei* 

 and a due degree of heat, as in most cases it is probable 

 that the oxygen is obtained by the decomposition of 

 that fluid. 



10. When a seed is sown under common circum- 

 stances, one of the first indications of the act of germi- 

 nation is a swelling of it from the moisture it imbibes, 



