THE METABOLISM OF PLANTS. 1 77 



therefore be directly transferred to the growing shoot : when 

 it is accumulated in the form of- proteid, as in the potato for 

 instance, the proteid is doubtless converted into amides, just 

 as in the seed, and these are used by the shoot for its nutri- 

 tion and growth. 



It remains for us to become acquainted with the mode in 

 which the plastic materials are used for the building-up of 

 the structure of the plant. We may take the growing-point 

 as the object of our study. We know that in a growing- 

 point a formation of new cells is taking place, that is, that 

 cell-wall and protoplasm are being produced. This produc- 

 tion must be effected at the expense of plastic materials 

 supplied to the growing-point from other parts of the plant. 

 But these plastic materials cannot be detected as such in the 

 tissue of the growing-point, the primary meristem (seep. 151). 

 Schacht, Sachs, and others have found that although starch 

 is usually present in considerable quantity in a growing 

 member, and can be traced into the immediate neighbour- 

 hood of the growing-point, yet it cannot be detected in the 

 cells of the growing-point itself. Sachs also failed to detect 

 sugar, and the various observers (Pfeffer, Borodin, Schulze, 

 de Vries) who have studied the distribution of amides in the 

 plant have likewise failed to detect them in the cells of the 

 growing-point. On the other hand, proteids are abundantly 

 present, and these are doubtless formed from the amides and 

 the carbohydrates (together with sulphates and perhaps phos- 

 phates) supplied to the cells of the growing-point. These 

 proteids serve as material for the construction of the proto- 

 plasm which is required in connexion with the processes of 

 cell-multiplication. The cell-walls are formed, according to 

 Schmitz and Strasburger, from portions of the protoplasm 

 (see p. 26). The processes going on in a growing-point are 

 then these : the cells are being supplied with plastic materials 

 from the cells lying behind them which contain these materials; 

 from these plastic materials proteid is constructed so that the 

 plastic materials themselves cannot be detected in the meri- 

 stematic cells ; from the proteid living protoplasm is produced, 

 and from a portion of the protoplasm cell-walls are formed. 



V. 12 



