2i8 General Botany 



most of the cell, cr bundles of microscopic needle crystals. If 

 you happen to have bitten into the corm of the common Jack- 

 in-the-pulpit and felt the stinging sensation in your mouth, you 

 have come in contact with the needle crystals, even if you have 

 not seen them. The crystals pierce the soft tissues of the mouth 

 and continue to irritate until they are dissolved. Oxalic acid 

 is produced in plant cells under certain conditions, and in the 

 presence of calcium it is precipitated as calcium oxalate (Fig. 28). 



The middle lamella, which holds plant tissues together, is com- 

 posed of calcium pectate, a compound of calcium and pectic acid. 

 The use of calcium in the formation of this compound is probably 

 the most important role of calcium in the plant, since no other 

 element can be successfully substituted for it in the building of 

 the cell wall. The presence of calcium in the soil also affects 

 the permeability of the cell membranes and thus facilitates the 

 absorption and retention of other salts by the roots. 



Calcium is also important in soils because it neutralizes acidity. 

 Red clover, alfalfa, and blue grass, for example, cannot withstand 

 acid soil conditions. This explains why lime and wood ashes are 

 recommended for improving lawns. Lime improves the texture 

 of many soils, and this in turn improves its drainage, water- 

 supplying power, and its aeration. 



Potassium is essential to the growth of plants, although we 

 know of few potassium compounds in plant tissues. Cell division 

 does not occur in its absence, and it plays an important role in 

 the chemical transformations that are continually being made in 

 the living cells among carbohydrates, organic acids, fats, pro- 

 teins, and other less familiar substances. Weak-stemmed plants 

 that occur in the absence of sufficient potassium appear to be due 

 to the need of potassium in the synthesis and translocation of 

 carbohydrates necessary for the formation of thick cell walls. 



Magnesium forms a part of the chlorophyll molecule and is 

 therefore indispensable to all green plants. It is also necessary 

 to the growth of non-green plants. 



