METABOLIC PROCESSES IN THE PLANT. 325 



quently happens, when any of the tubular cells are accidentally 

 opened, that these mucilaginous masses with the raphides escape, 

 so that we observe them outside the section in the surrounding 

 fluid. If we treat the sections with potash or Acetic acid, the 

 raphides do not dissolve. We further prepare transverse sections 

 from the leaf of Beta vulgaris. On microscopic examination we see 

 clearly the epidermis of the upper and under sides of the leaf, the 

 slightly developed palisade parenchyma, and the spongy paren- 

 chyma, with its numerous intercellular spaces. In this last 

 occur the so-called granule tubes, cells which are filled with 

 small crystals of Calcium oxalate. We prepare still further a 

 section from a shoot about 5 mm. in thickness of Tilia parvifolia. 

 The appearance of the transverse section has already been de- 

 scribed in 42. We need only notice here that in the outer part of 

 the medullary rays, and in the tissue of the primary cortex r many 

 cells are present which contain cluster crystals of Calcium oxalate. 



Some plants, especially the Crassulacere (e.g. Sempervivum, 

 Bcheveria, Bryophyllum) are distinguished by the fact that their 

 cell-sap contains very large quantities of Malic acid, which, as 

 Kraus l proved, is for the most part combined with lime. The 

 malate, Avhich is soluble in the cell-sap, sometimes constitutes 50 

 per cent, of the dry weight of the : sap in the leaves of these plants. 

 We pound a few leaves of Bryophyllum in a mortar, transfer the 

 pulp to a dry filter, and determine the dry weight of a small part 

 of the sap obtained, the larger part of it being mixed with 4 or "> 

 times its volume of 96 per cent, alcohol. The malate separates out 

 in the form of a white powdery precipitate, which "we can filter 

 off, wash, dry, and weigh. 



It is very often important in plant physiology to determine the 

 acidity of plant saps, i.e. the amount of titratable acids which 

 they contain. According to circumstances, different methods 

 must be employed, and we will now consider these. The method 

 of preparing the saps or extracts in which the quantity of acid is 

 to be determined is a matter of prime importance. If in com- 

 parative work it is only desired to determine relative .values 

 for the acidity, the material when very juicy, as is the case, e.f/., 

 Avitli rhubarb leaf-stalks, is rubbed on the grater, or if poor in 

 water is cut up into pieces and pounded as thoroughly as possible 

 in a porcelain mortar. The pulpy masses are squeezed in a cloth 

 by the hand or by means of a press, with as uniform a pressure 

 as possible, and then the sap is finally cleared by filtering. Under 



