GROWTH 



209 



the straw rose above it, a second pipe was added on the 

 top of the first, and so on. It is clear that under such 

 circumstances the plant received light only from above, 

 being always shaded at the sides. As was expected, these 

 conditions produced very high, weak, and flexible 

 straw. On submitting this straw and normal healthy 

 straw to a comparative microscopic investigation, the 

 following difference was observed : the former had per- 

 ceptibly longer 

 cells, but the 

 walls were con- 

 siderably thinner 

 than those of the 

 latter, the cells 

 of which were 

 shorter and had 

 thicker walls. It 

 follows that light 

 does not, so to 

 speak, retard 



growth, but apparently only modifies its direction. 

 Instead of growing in all directions, the cell -walls 

 thicken. The same difference has been observed when 

 healthy straw is compared under the microscope with 

 laid straw, as may be seen by comparing the transverse 

 sections of two pieces of straw, one of each kind, in fig. 61 . 

 All the cell -walls of the normal one (on the right- 

 hand side) are much thicker, and the lower rows (the 

 external layers of the straw) are even so thick that 

 the cavity is reduced almost to a point. (The lines 

 in fig. 61, joining the cavities of adjoining cells, are the 

 pits.) As well as having their walls thus thickened, 

 the cells of the straw of a normal specimen are con- 

 siderably shorter in longitudinal section. Therefore it 

 is in a too rapid elongation of the straw, together with 

 an insufficient thickening of the cell-walls caused by 

 mutual overshadowing in a thick crop, that we must 



o 



FIG. 61. 



