14 PLANT PHYSIOLOGY 



are the green chloroplasts. Within the cytoplasm may also be seen, 

 with properly stained material, other bodies called mitochondria 

 or chondriosomes. It seems probable that some of these chon- 

 driosomes may be the originators of the plastids and develop 

 into them. 



Within the cell, apart from the protoplasm itself, may be various 

 amounts of water with salts, sugars, and other substances in solu- 

 tion. These may be materials which are to be used up in later 

 constructive processes or waste products which have resulted 

 from previous operations. This solution is called cell sap, and 

 the regions containing it are called vacuoles. The cell sap in addi- 

 tion to matter in solution may also contain crystals of various 

 kinds such as calcium oxalate as well as some starch grains and 

 other insoluble materials. 



Although Lepeschkin (1930) doubts their importance (and 

 even their existence in many cases), most observers are agreed 

 that the nucleus, plastids, etc., as well as the cytoplasm, have 

 membranes on their inner and outer surfaces, which permit cer- 

 tain substances to pass through while keeping others from doing 

 so. Such membranes are said to be differentially permeable, and 

 the reactions which take place at such surfaces are extremely im- 

 portant in regulating the activity of the cell. The cell is able to 

 carry on many different processes at the same time owing to these 

 various membranes which surround the different structures in 

 it, but the cell is nevertheless the physiological unit of the organ- 

 ism just as it is the morphological one. The sum total of the ac- 

 tivities of the organism is the sum total of the work of the cells. 



The Colloidal Condition 



Graham.— Thomas Graham, an English physicist, who was 

 working on the diffusion of substances through parchment paper 

 (1861-1864) noticed that certain materials such as salts, acids, 

 sugar, and the like easily diffused through the parchment mem- 

 brane; while starch, albumin, gelatin, etc., did not pass through. 

 He says: "As gelatine appears to be its type, it is proposed to 

 designate substances of the class as colloids (Gr. kolla = glue) and 

 to speak of their peculiar form of aggregation as the colloidal condi- 

 tion of matter. Opposed to the colloidal is the crystalline condition." 



Although Graham speaks of the "colloidal condition" he seems 

 to have regarded colloids as quite distinct from crystalloids and 



