INTRODUCTION 7 



of life. Obviously, then, a knowledge of the chemical nature of protoplasm 

 will be of primary importance to us, but first of all, we may take note of some 

 of the more important points in the chemistry of the cell-wall and cell-sap. 



To start with, the cell-wall is obviously not a chemical entity, for in addition 

 to the carbohydrates, of which it is in the main composed, it contains mineral 

 matters and water as well. The carbohydrates which enter into its composition 

 are polysaccharides belonging to the cellulose group. It is rarely the case 

 that the cell-wall is composed of a single chemical compound ; far more com- 

 monly many closely related bodies enter into its composition. The water, 

 which is always present, is not, however, located in definite spaces in the wall, 

 but occurs as ' water of imbibition ', that is to say, in a state of minute sub- 

 division between or within the ultimate molecules of the substance of the 

 cell-wall itself. This is not the appropriate place to enter into the study of 

 the subject of imbibition as a whole (Lecture XXXII), but we may note the 

 following points of interest. In the first place, no definite and stable chemical 

 union exists between the imbibed water and the substance of the cell-wall, 

 since it is possible to squeeze the water out, at least in part, by mere mechanical 

 pressure, or to permit it to evaporate into the atmosphere. What is still retained 

 in the wall after pressure or evaporation may be driven off by heat. If the 

 desiccated cell-wall be once more brought into contact with water, it absorbs 

 it again with considerable force, and in quantity dependent directly on tempera- 

 ture. Simultaneously with the absorption of the water the volume of the wall 

 is increased, and an important physical alteration takes place also in the swollen 

 mass. Just as a piece of glue, hard and brittle as it is in the dry condition, 

 becomes soft and pliable when wet, the cell-wall alters in character when 

 similarly treated. For example, a fresh stem of Cohaea scandens (a well-known 

 climber) can be twisted round one's finger like a piece of string, but if dried 

 it becomes as brittle as glass. The alteration effected in the character of the 

 ceU-wall by the imbibition of water cannot be other than of the greatest 

 significance in plant economy. 



The mineral matters occurring in the wall may be in part dissolved in the 

 water of imbibition, but most of them are distributed in the solid form in 

 a minute state of subdivision among the particles of carbohydrate. 



The cell-sap consists for the most part of water, in which are always 

 dissolved a large number of organic and inorganic compounds, while there 

 are also present in addition sohd particles in suspension, resulting from the 

 precipitation of substances normally soluble in the sap. 



Microscopic investigation of the protoplasm reveals the presence of a hyaline 

 ground substance, in which are imbedded the special organs of the protoplasm 

 already referred to under the names of chloroplast and nucleus, as well as 

 a number of granules and vesicles (microsomata), some of known, some of 

 unknown composition. The varied streaming movements exhibited by this 

 ground substance, or hyaloplasm, shows that it contains much water ; indeed, it 

 is easy to demonstrate directly its presence in the protoplasm. We may further 

 assume that the relationship of the water and the protoplasm is similar in 

 character to the relation subsisting between the water and the cell-wall ; in 

 other words, the protoplasm may be regarded as a swollen body. So far then 

 as the hyaloplasm itself is concerned, we may consider it, when saturated 

 with water, as a complex of organic nitrogen- and sulphur-containing bodies, 

 or proteids, and looked at from this point of view it has often been the custom 

 to regard protoplasm as proteid in solution. Because the cell-sap may also 

 contain dissolved proteid, and because this proteid when removed from the 

 plant no longer exhibits that vitality which makes protoplasm so interesting, 

 a distinction was made between dead and living proteid, and the name proto- 

 plasm was confined to the latter. The chemical investigation of as pure 



