126 THE PHYSIOLOGY OF PLANTS 



especially of the white varieties of potato. Indeed, the number 

 of these protein crystals can give us some clue to the amount 

 of starch present in the potato, for it has been found that 

 the greater the quantity of these crystals the poorer the tuber 

 is in starch. Fig. i 8 rej^resents a piece of a section through 

 the cortex of a potato. The actual peel (k) consists of a 

 number of cork cells which give it its toughness. Below the 

 cork cells are the outermost cortical cells (pi), which are rich 

 in albumens and poor in starch ; here we shall find the most 

 protein crystals (cr). The further we proceed to the centre, 

 the lai'ger the amount of starch contained in each cell. These 

 starch grains (s) have an oval shape, and have a very charac- 

 teristic and peculiar stratification. 



Very often the crystalline albumens {crystalloids) occur 

 within other solidly formed albumens. In the seeds of 

 phanerogams they occur in the form of aleurone grains, and 

 form the reserve albumens which are used up on germination. 

 In these aleurone grains, which in some plants (Pceo7iia, 

 Ricinus) are soluble in water, some albuminous compounds 

 are mixed with phosphates of lime and magnesia, and assume 

 a spherical form (globoids). 



We see, therefore, that the albumens occur like the carbo- 

 hydrates in form of insoluble reserve material, and in soluble 

 transportable substance. 



We must here make mention of the groups of alkaloids, 

 characterised also by the fact that they all contain nitrogen. 

 They have organic bases, containing nitrogen, carbon, hydrogen, 

 and generally also oxygen, and are probably first formed by 

 the protoplasm, and contained in the cell sap, but are often 

 found in the cell-walls too. Among the alkaloids devoid of 

 oxygen we have trimethylamin, which gives the peculiar odour 

 to the ergot of rye, and some of the members of the Goose- 

 foot family, &c. 



The oxygen- containing alkaloids comprise some of our most 

 valuable drugs, such as strychnine, quinine, morphine, atropine, 

 which occur combined with acids in the various organs of 

 certain plants. 



