180 Mississippi Valley Horticultural Society. 



selves changed in respect to composition or hardness. They have simply ar- 

 ranged themselves in regular and fixed order, like the bricks of a tower, and 

 a crystal is the result. To attain this arrangement more or less of molecu- 

 lar movement is required, and anything whatever that tends to prevent this 

 movement tends to prevent the water freezing at the temperature stated. 

 Indeed, only pure water freezes at the degree marked on .the thermometers. 

 If a little salt or sugar is added, the temperature must be below 32° Fahr. for 

 crystallization to take place ; and the lower, the greater the proportion of 

 salt or sugar. Water, saturated with salt, may be cooled down to 4° Fahr. 

 before ice is formed. When ice is produced on such a solution, it is, as be- 

 fore, the arranged molecules of water which form the crystal, the foreign 

 molecules being excluded from the structure. The ice is therefore pure wa- 

 ter, save that impurities may be mechanically caught and held among the 

 crystals. 



Passing now to another series of facts pertaining to the structure of the 

 solid parts of organic bodies, it may be first stated that water fortiis an essen- 

 tial part of the texture. In plants, with which we are now concerned, all 

 the solid parts are composed of cells usually only to be seen with the micro- 

 scope. These cells have walls or sack-like membranes which often enclose 

 various substances more or less mixed with water. Sometimes the cell cav- 

 ities are full of liquid water, forced up from the earth by the roots. But 

 aside from this liquid water contained in the cells^ the molecules of water 

 help form the solid parts, as of the cell-walls. This last is perhaps diflicult 

 to comprehend, but it is exceedingly important that we should understand 

 the fact in order to rationally acquaint ourselves with what takes place when 

 a plant freezes. It has already been said that water is really made of minute 

 solid particles called molecules. The substance of the cell-walls, known as 

 cellulose, is likewise composed of molecules, but of more complex structure 

 and undoubtedly of considerably greater size than those of water. In the 

 natural composition of the cell-wall the cellulose molecules may be repre- 

 sented by the bricks in masonry, and the water molecules by the grains of 

 sand in mortar. Between these different kinds of molecules there is a strong 

 attraction or adhesion which binds the whole into a solid substance. There 

 is plenty of water present, but no liquid. The water molecules are as tiuly 

 a part of the structure as are the cellulose molecules. Ripe seeds have no 

 liquid water in them, nothing but this water of structure, and comparatively 

 little of that; so of the other parts of many plants in certain normal condi- 

 tions of their existence; while, on the other hand, more than nine-tenths of 

 the weight of rapidly growing shoots is water in both li(]ui(l and molecular 

 states. 



Let us remember that every solid is made up of invisibly small molecules, 

 and that these are held together by the attraction that exists one for another; 

 that the force or eflect of this attraction varies inversely as the square of the 

 distance. This distance, at most, is so minute that av;/ variation makes con- 

 siderable, perhaps very much, difference in the result. Il is this molecular 

 attraction that binds the cellulose molecules together witli the water mole- 



