THE FOOD OF PLANTS. 89 



Ammonium Magnesium phosphate (in examining sections they 

 are produced in the cells) have very characteristic forms. 



Very searching investigation is still required as to the dis- 

 tribution of organic and inorganic compounds of mineral sub- 

 stances. But still some results have been obtained. 



In resting seeds (we examine sections from soaked seeds of 

 Phaseolus and Ricinus), Phosphoric acid, lime, and potash 

 occur in organic compounds, and so are detected not directly 

 in the sections, but in the ash which these yield. In germination 

 these organic compounds are split up. Phosphoric acid, lime, 

 and potash travel as inorganic salts in the parenchyma of the 

 cortex and pith, both of stem and root, and also in the par- 

 enchyma of the leaf nerves. In all these tissues the above 

 substances can therefore be found directly (not merely after 

 incineration). For example, the parenchyma of maize seedlings 

 is rich in potash and phosphoric acid. 



In the growing points and in the mesophyll of seedlings occur 

 large quantities of organic Phosphorus compounds, while in- 

 organic compounds of Phosphorus are entirely wanting. The 

 Phosphoric acid can hence be detected only by examination of 

 the ash. The growing points and the mesophyll are thus the 

 limits of migration, places of formation of Phosphorus-con- 

 taining organic bodies. The parenchyma of cortex, pith, and 

 leaf-nerves, on the other hand, forms the channels in which 

 the mineral substances travel during germination. 



It may be remarked further that the growing points, the 

 cambium, and the sieve tubes (we may e.g. test the sap of the 

 sieve tubes of Cucur-bita, and also the ash which it yields) of 

 seedlings and grown plants are almost or entirely devoid of lime, 

 while they contain more or less large quantities of potash and 

 Phosphoric acid, chiefly in the form of organic compounds. 



To familiarise ourselves with the method of testing micro- 

 chemically for iron, we soak seeds of Sinapis alba in water, then 

 remove the seed-coat, and lay the embryo, which, as is well known, 

 bears folded cotyledons, in a 2 per cent, solution of yellow prussiate 

 of potash (Potassium ferrocyanide). After half an hour we 

 transfer it to 20 per cent. Hydrochloric acid, wash after some time 

 with distilled water, and spread out the cotyledons on the slide.* 

 Only the primordia of the vascular bundles of the cotyledons (and 



* Steel needles naturally must not be used. 



