210 THE LIFE OF THE PLANT 



look for the real reason of the laying of crops. This bad 

 effect can be prevented by sowing the seed more thinly, 

 or by drilling. Then every plant will get sufficient 

 light for its normal development. 



These facts lead us to contradict one more current idea 

 about the growth of the plant. We have already seen 

 that growth does not always imply increase in matter — 

 during germination the increase in volume is accompanied 

 by a loss of substance. We now see that growth does 

 not always imply the elongation or expansion of an organ, 

 because sometimes growth can show itself in a different 

 way — in a thickening of the cell-walls. Strictly speak- 

 ing, we see that a plant grows only when its cell-walls 

 grow, whether in length, in breadth, or in thickness. 



But it is not only light which may modify the form 

 of cells. Occasionally this is also possible as the result 

 of strictly mechanical causes. We have seen in our last 

 lecture that so-called annual rings are noticed on the 

 transverse section of the trunk of a tree (fig. 44, HI). 

 These rings are most clearly marked in fir-trees — in a 

 pine, for instance. The reason for such rings appears 

 to be connected with the periodical cessation of the 

 vegetative processes during the winter. Neverthe- 

 less, if in spring, i.e. after the winter's rest, new rows 

 of quite similar cells were added to the cells deposited 

 the previous autumn there would be no difference 

 whatever between the adjoining layers ; the boundary 

 between them would be imperceptible, and they would 

 all blend into one continuous mass. The layers are 

 clearly marked only because the wood formed in spring is 

 distinctly different from the wood formed the previous 

 autumn. Even with the naked eye we may see that 

 two parts can be differentiated in every annual ring : 

 the spring or early wood which lies nearer the pith, and 

 the autumn or late wood which lies nearer the bark ; 

 the former is looser and therefore lighter in colour, 

 the latter is denser and consequently darker. We notice 



