AMERICAN HONEY PLANTS 195 



substance. A considerable part of the thickening of matured cell-walls 

 has been laid down on the original partition between two cells, and not 

 only differs from this but is not alike in diflerent kinds of cells, and in 

 structures like wood and cork it is impregnated with other materials that 

 affect the cell-wall very greatly in such respects as hardness and permea- 

 bility to water. 



The shells of nuts, for instance, are so impervious that they are com- 

 monly "stratified" by planters so that their hard shells may disintegrate 

 more or less as a preliminary to germination, a process that not infre- 

 quently requires more than a year unless hastened by some expedient like 

 that of passing haw fruits through the digestive mill of poultry as a means 

 of softening their boney cores, of filing the hard envelope, which is a 

 favorite trick of gardeners with nut-like fruits of the lotus or with canna 

 seeds. (This is similar to the scarifying of sweet clover seed. — Editor.) 

 This is the reason that several times as much clover seed — even good 

 seed — must be used on an acre as seems necessary for securing the de- 

 sired number of plants. One of these modifications is usual in the outer 

 byers of cell-walls on the surface, and it is called cuticularization. Cuti- 

 cularized walls are more or less completely water-proofed. When the cells 

 that produce nectar are at the surface, their outer walls are cuticularized 

 in this way; when they are within the nectary and the nectar passes out 

 through stomata, this is scarcely, if at all, the case. 



The greater part of nectar is water, which reaches the surface from 

 within the plant cells. To do this it must pass through walls that are 

 little, if at all, cuticularized, or it must break through the cuticle. This 

 does not mean that it must break through the entire cell-wall; a small 

 part if this is modified by the protoplasm into a gum or mucilage, or some 

 similar substance, and the water accumulates in this layer and swells it 

 until the overlying cuticle is burst. Some form of sugar is a frequent 

 result of this disintegration of cellulose. Dissolved sugars pass through 

 the ordinary cellulose wall, but they do not pass through the ordinary 

 surface layer of protoplasm in the outer cells. 



When water is separated from a solution like that of sugar by a filter 

 of this sort, which allows water to pass but is not permeable to the dis- 

 solved substance, the action is set up that physicists call osmosis, and 

 water accumulates on the side of the dissolved substance until it exer- 

 cises a very considerable pressure. This action not only bursts the cuticle, 

 v/hen it starts beneath it, but results in a flow of water within the plant 

 at that point. 



The absorbing roots of plants show another result of this physical 

 property, osmosis. They are not waterproofed; water is continuous 

 through them, from the thin layer in which it occurs about particles of the 

 sell, to the water which composes a great part of the weight of the proto- 

 plasm within the cells. This sap of the root cells contains dissolved sugar 

 and other osmotic substances. Osmotic absorption by the roots results in 

 ^ pressure of several atmospheres. This pressure, passed from cell to cell, 

 ^ives the crispness to fresh celery. Its loss, through evaporation from the 

 leaves, results in the loss of this crispness, or wilting. 



