STRUCTURE AND ARRANGEMENT OF LIVING MATTER 219 



that it takes a part in the processes of formation of proteids, car- 

 bohydrates and fats. It sustains an important physical function 

 in the maintenance of turgidity. 1 



Magnesium, is taken up in all of its salts except the chloride 

 and is found in globoids, occurring in the greatest abundance in 

 seeds, also in embryonic tissues. It is concerned in the synthetic 

 processes although its exact office may not be delimited. 



Iron may be absorbed in almost any of its salts, but is used 

 only in minute quantities. It is present both in the plasma and 

 wall and may enter into some organic unions. In this form it 

 may aid in the construction of chloroplasts. Its presence is nec- 

 essary for the formation of chlorophyl although it does not enter 

 into chemical union with this compound. It is equally indispen- 

 sable for species which do not construct chlorophyl. 2 



288. Structure and Arrangement of living Matter. A cell or 

 protoplast is a minute mass of protoplasm which tends to assume a 

 globular form, but which undergoes such modifications by growth, 

 differentiation, internal movement and mechanical pressure that it 

 may assume almost any form from spindle-shaped, tabloid, etc., 

 to globose. Protoplasm is a viscid translucent substance, which 

 in some instances appears to consist of a meshwork or reticulum 

 enclosing a ground substance (hyaloplasma). The reticulum 

 shows more or less abundant rounded bodies on its branches 

 (microsomes), granules, and imbedded in the mass are numbers 

 of inert substances such as crystalloids, giving rise to the " fibril- 

 lar theory " of the structure of living matter. To other observers 

 protoplasm has appeared to show a foam structure consisting of 

 minute closely crowded drops of " alveolar substance " imbedded 

 in another liquid constituting an emulsion (alveolar theory of 

 Butschli). Protoplasm is supposed by some investigators to con- 

 sist of innumerable minute granules which form its essential basis, 



1 Copeland, E. B. The relation of nutrient salts to turgor. Bot. Gazette 24 : 399. 

 1897. 



1 Pfeffer. Plant Physiology. 1 : 410. 1900. 



Loew, O. Physiological role of mineral nutrients. Bull. No. 18, U. S. Dept. of 

 Agric, Division of Veg. Path, and Physiol. 1899. 



