28 LECTURE II. 



The Vacuole and the Cell- Sap. 



In a very young cell the protoplasm and the nucleus 

 fill the whole cavity so that no vacuole is apparent. At 

 an early period lacunae containing watery fluid make their 

 appearance in the protoplasm ; then, inasmuch as the growth 

 of the protoplasm does not keep pace with that of the cell-wall 

 and inasmuch as the peripheral portion of the protoplasm 

 always remains in direct contact with the cell-wall, these 

 lacunae become larger and fuse so as to form one continuous 

 vacuole which is traversed here and there by bands of proto- 

 plasm which connect the peripheral layer (primordial utricle) 

 with a more or less centrally placed mass. It occurs, not 

 uncommonly, that ultimately the whole of the protoplasm is 

 required toform the primordial utricle, and in such a case 

 the nucleus is parietal. 



The cell-sap, the watery fluid which saturates the proto- 

 plasm and the cell-wall and occupies the vacuole, consists 

 of water holding in solution a number of both organic and 

 inorganic substances, which have either been formed and 

 thrown off by the protoplasm, or have yet to be absorbed 

 and elaborated by it. The principal organic substances are 

 the following; a substance which reduces alkaline cupric 

 solutions, to which the general term " sugar" may be applied ; 

 organic acids, either free or in the form of acid salts, for the 

 cell-sap reddens litmus-paper : colouring matters, in the cell- 

 sap of many cells : crystallisable nitrogenous bodies, such as 

 asparagin, leucin, tyrosin, especially in organs in which meta- 

 bolism is active. The inorganic substances are probably 

 salts of potassium and sodium, chiefly nitrates, chlorides 

 and sulphates. 



The principal forms of sugar and its allied bodies (carbohydrates) 

 which occur in the cell-sap are : cane-sugar (sucrose or saccharose), 

 C M HOn), which reduces the alkaline cupric solution only after prolonged 

 boiling: mannite (C 6 H 14 O 6 ) and inulin (C 6 H 10 O 5 ) which do not reduce the 

 alkaline cupric solution ; inulin is deposited on treating the tissues with 

 alcohol, in the form of sphserocrystals : glycogen (C 6 H 10 O 5 ), which forms 

 an opalescent aqueous solution, and does not reduce the cupric solution 

 on boiling : glucose (C 6 H 13 O 6 ), which readily reduces the alkaline cupric 



