254 EXPEFTMFjyTS WITH PLANTS 



phosphorus) ; it is probable that in this process oxalic 

 acid is formed, which unites with lime to form the 

 numerous crystals of oxalate of lime which are seen in 

 the neighborhood of the bud: they are easily observed 

 with the low power of the microscope (see also Fig. 

 135, cr). The sugar may be changed temporarily into 

 starch, not only in the growing region but in the cells 

 through which it travels to get there; and for this 

 reason it is easy, in most cases, to trace its path by 

 the application of iodine solution to the cut surfaces 

 of a stem divided lengthwise. 



To trace the proteid substances may not be so easy 

 unless we have a favorable plant, like the Squash or 

 the Pumpkin. On cutting across the stem of a Pump- 

 kin, the proteid substances at once ooze out at certain 

 spots and coagulate. The stem may be laid for a time 

 in alcohol (to coagulate the proteid and extract the 

 chlorophyll), and the nitric acid test (also the sul- 

 phuric acid and sugar test described on page 166) may 

 then be applied. In less favorable cases iodine may be 

 applied; this turns proteids brown (not blue or black). 

 (It may be necessary to use a hand -lens or a com- 

 pound microscope.) Our examination of the Squash 

 stem shows us that the proteids are contained princi- 

 pally in the bast. The bast lies on both sides of the 

 wood and is composed, like the wood, of large and 

 small cells. The large, wide cells, called sieve-tubes 

 {st^ Fig. loel), are, as we see in sections, really long 



