[26 



. B01 VNY OF THE LIVING PLANT 



P ITEIN S HESIS. 



Proteins and carbohydral re the most important constituents 

 of the plant body, though there is In addition a great range of sub- 

 B tan >ther typ< •- we saw in Chapter III., the especial im- 



portan proteins lies in the fad that the living substance, proto- 



plasm, i- largely made up from them. Chemical analysis shows that 

 proteins and their derivatives may account for as much as 60 per 



•it. of the dry weight <<t protoplasm. 



The protein molecule contains the elements carbon, hydrogen, 

 oxygen, nitrogen and sulphur (the latter in very small proportion). 

 It ia an dingly complex structure with a molecular weight 



running into many thousands. 



Ji 



Fig. 84. 



A great many different pro- 

 teins have been detected in 

 plants, and each plant con- 

 tains a variety of them. They 

 are all built up in the main 

 from Amino -acids. These are 

 organic acids containing the 

 amino group, - NH 2 . Glycine, 



A, cell from the endosperm of Ricinus in water, Acnarrir Arirl Trwr^rnT^ricm^ 



which causes the outer coat of the aleurone grains to ^P^ rL1( - AUU, irypiopnane 



swell. B. is dated .ilcurone grains in oil. A: = albumen „ nr l fVQfinp mo\r hp nampH 



tab. g globoid ( > 540.) (After Strasburger.) dnu ^yatme mav De namea 



as examples. Some of these 

 amino-acids are themselves quite complex, but it is probable that 

 several hundreds of amino-acid molecules are required for the forma- 

 tion of a single protein molecule. Certain proteins, distinguished 

 as iVwc/Voproteins, contain phosphorus in addition to the above 

 elements. The nuclear structures of the cell are believed to con- 

 sist largely of these. As mentioned in Chapter III., proteins give 

 colloidal solutions, and it is doubtless in this state that proteins are 

 t-nt in the protopla-m. 

 In addition to the protoplasmic proteins there are also reserve 

 ems, regularly presenl in seeds and other storage organs either 

 in a crystalline form (Fig. 81), or in the form of grains, known 

 Aleurone grains. These may be quite complex in structure, as 

 t'»r example in the Castor Oil seed (Fig. 84). Here the aleurone 

 in contains within an outer coat of protein a protein crystal, and 

 a so-called globoid consisting of protein associated with inorganic 

 groups. Enzymes that are capable of resolving proteins into their 

 constituent amino-acids are presenl in plant tissues: this process 



