238 BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 



570. " The contrast between assimilation and respiration ^ is very- 

 marked : one is substantially the opposite of the other. The follow- 

 ing tabular view displays the essential differences between them : — 



Assimilation Proper 



Takes place only in cells contain- 

 ing chlorophyll. 



Requires light. 



Carbonic acid absorbed, oxygen 

 set free. 



Carbohydrates formed. 



[Energy is stored.] 



The plant gains in dry weight. 



Respiration 

 Takes place in all active cells. 



Can proceed in darkness. 

 Oxygen absorbed, carbonic acid 



set free. 

 Carbohydrates consumed. 

 [Energy is brought into use.] 

 The plant loses dry weight." 



571. Resting periods. — The dormant condition of seeds and buds 

 has already been described. In the periods of suspended animation 

 respiration is reduced to its lowest limits. Some seeds may be kept 

 for years without loss of vitality. We must suppose that all the while 

 the protoplasm is to a very slight extent active, and that feeble respi- 

 ration is going on. 



572. Growth. — Were we to trace the inner and outer changes that 

 lead to the formation of a complete leaf, — taking the leaf as an 

 example of the organs in general, — w^e should find the following 

 course of events. First a slight prominence is to be seen close to the 

 tip of the stem. This elevation is caused by the rapid multiplication 

 of the cells at the point where the new leaf is to appear. All the cells 

 at this point are capable of dividing ; the tissue is said to be embry- 

 onic. Of course division is accompanied by the increase in size of the 

 cells produced. As the protuberance grows, it soon shows some signs 

 of external shaping. Lobes appear, if the mature leaf is to be lobed 

 or compound. But the whole mass of cells remains embryonic in 

 character, and the cells are still relatively small, until the new organ 

 has been formed and shaped into something like a miniature of its 

 mature condition. Then another phase of growth sets in. Few new 

 cells, or none, are made, but all the cells begin to elongate and enlarge. 

 As a result the w^hole leaf expands, and it may do so very rapidly. 

 This phase — the phase of elongation in groictli — is seen in the swift 

 expansion of foliage from winter buds in spring. Finally, as full size 

 is being attained, a third phase appears. The cells of the leaf indi- 

 vidually take on their characteristic forms, by final changes in shape 

 and in the nature of the cell walls. 



573. Three phases are thus to be made out in the growth of any 

 organ : (1) the formative, or embryonic phase ; (2) the phase of elonga- 

 tion ; and (3) the phase of internal development. But it is not to be 



1 From Goodale's "Physiological Botany," p. 356. 



