240 VEGETABLE PHYSIOLOGY 



the cell in which it lies, and not occupying a definite space 

 as do aleurone and starch grains. Whether it is secreted 

 from the substance of the protoplasm, or whether the 

 materials of which it is made are taken to the latter in a 

 state near the condition of the finished fat, is uncertain. 

 It is formed by the combination of a fatty acid with gly- 

 cerine. Both these bodies can be formed in the plant, but 

 how they are finally presented to us in the shape of oil is 

 still in need of elucidation. As the oil appears in the cell 

 it seems to point to a process of breaking down of the 

 protoplasm itself, and not to a direct combination of the 

 antecedents mentioned. If we stain cells which are form- 

 ing fat with osmic acid, which colours fatty bodies brown 

 or black, we see in the protoplasm small specks of fatty 

 matter, which, while in the youngest cells mere dots, are 

 in older ones larger, and can be recognised as droplets. In 

 still older ones the blackness permeates the whole proto- 

 plasm, indicating that the latter is saturated with the oil, 

 the droplets having run together in consequence of their 

 number and dimensions. 



The appearances are, however, not inconsistent with 

 the view that the work of the protoplasm is only to effect 

 the ultimate changes or interactions of the glycerine and 

 the fatty acids which are transported separately to the cells 

 or perhaps formed there from some antecedent. 



The deposition of fat in some cases, particularly in 

 leaves, has been stated to be effected by the agency of 

 certain plastids corresponding to the leucoplasts already 

 mentioned in connection with the formation of starch 

 grains. These structures, which have been called ela'io- 

 plasts, are curious bodies of various shapes, sometimes 

 round or oval, sometimes irregular in contour, which lie 

 near the nucleus of the cell. Like the other plastids they 

 consist of a spongy protoplasmic framework, in the meshes 

 of which the oil is formed, much as it is in the protoplasm 

 of the seeds already described. 



All these bodies, when acted upon by a process ana- 



