32 BOTANY OF THE LIVING PLANT 



There is no doubt that chemically and physically protoplasm is very 

 complex indeed, and the study of its nature is attended by great 

 difficulties. Under the microscope the cytoplasmic part of the 

 protoplasm has the appearance of a clear medium with many sus- 

 pended granules. Experimental evidence shows that protoplasm is 

 usually liquid in nature, though considerably thicker or more viscous 

 than water. In some cases the consistency is more that of a jelly. 

 The nucleus and plastids are somewhat denser parts of it. 



Chemical analysis has indicated that protoplasm is a heterogeneous 

 mixture of a great variety of chemical substances, the Proteins being 

 present in greatest quantity, both in the nucleus and the cytoplasm 

 (see Chapter VIII.). Associated with them are fats and their deriva- 

 tives, and mineral salts. All these substances are dissolved or sus- 

 pended in the water which constitutes over 80 per cent, by weight 

 of protoplasm. The constituents of protoplasm are to a considerable 

 extent in a colloidal condition, and some knowledge of the special 

 properties of colloidal substances and solutions is absolutely essential 

 to a consideration, however elementary, of the living cell ; for there 

 is no doubt that it is largely the colloidal nature of its constituents 

 that endows protoplasm with its remarkable powers. The cell-wall 

 and some of the metabolic substances occurring in cells are also 

 colloidal in nature. Graham, in his researches of the middle of last 

 century, divided soluble substances into two classes, as the result of 

 his studies on dialysis, — the term applied to the passage of substances 

 in solution through membranes such as parchment. He found that 

 certain substances passed freely through such membranes, and since 

 generally speaking they could also be readily obtained in a crystalline 

 condition he called them Crystalloids. As examples we have sugars 

 and soluble salts. Other substances were found not to dialyse, or 

 only very slowly, while a further character was their amorphous 

 nature when dehydrated. These he termed Colloids. Many plant 

 and animal products are of this type. As examples gelatin (a protein), 

 agar-agar, gums and starch may be quoted. These substances give 

 a special type of solution in water, known as a colloidal solution. In 

 such a solution the substance is present in the form of relatively 

 large particles, either because the molecules are aggregated into 

 groups, or because the individual molecules are very large ; or again 

 because they take into association a number of molecules of water. 

 Such solutions are intermediate between the true solutions given by 

 crystalloids, and suspensions or emulsions. The particles are per- 

 manently suspended in the water, they are invisible under the highest 



