327 
part played by each substance in the plant cell. Thus we know now 
that phosphorie acid is essential for the formation of ears, 
substances in the plant, sroga the albumens, whi 
fundamental importance in the transformations of substances in 
plants, result from an toraa iie E pim ic a compound, as 
is indicated by the regular occurrence of lecythin in protoplasm. 
Again, iron is an essential constituent of chlorophyll and sulphur 
of albumen, and hence must be supplied to plants. The true 
eicit A calcium was for long doubtful ; its action is now 
kn of a medicinal character, since it serves to neutralize 
e alw n edia 
product of the oxidation of the carbohydrates. It was formerly 
thought that mien fulfilled some important function in the 
leaves, being chiefly found in the foliage of plants. Since, how- 
ever, the leaves are also the chief seat of the oxalic acid, this 
distribution of the calcium is easily explained. 
The part played by potassium has only within the last three 
years been explained by Hellriegel, who, by exact experiments 
with beet-root showed that the amount = m in the beet 
stands i e close relation to the amount of potassium provided for 
the pla . Wagner has made the interesting observation that 
the ore a may si partly replaced by sodiu 
The exact value of magnesium to plants is not yet well under- 
stood, but it appears to be of importance in the formation of the 
nitrogenous substances of ee as in these considerable quantities 
of magnesium phosphate o 
Nitrogen is an indispensable plant-food, for it is an essential 
constituent of album 
In addition to the quantities of mineral substances required by 
plants to enable them to exhibit a healthy growth, further 
quantities are found to be essential to satisfy what has been 
termed, though not very aptly, the ** mineral-hunger " of the plant. 
This is best explained by an example. E. Wolff found that for 
the SH dente wd 100 parts of € sees edes 5 parts 
phosphoric acid were necessary en the remaining mine 
ubsta 
substances were necessary for the production of 100 parts of 
oat-plant :— 
Phosphoric acid ad a . ‘oO parts 
otash  ... pi ia eee av Oe 
Lime 3 T is Ps sx. OS 
Magnesia * vus a 22990 ay 
Sulphuric acid ... sé H. sco ON 
1:95 parts 
A total of 1-95 parts of mineral substances is us e ine y 
in the case of the oat-plant. However, there is oat-plan 
nature Shick contin so little as 1:95 per cent. The E pea 
is 3 per cent. The difference, 1:05 per cent., is the measure of the 
