INTRODUCTION 251 



thus to be exposed to the sunlight in a thin layer, and we may regard the 

 flattened form of the leaf as specially adapted to fulfil this function. SACHS 

 (1882, Vorlesungen fiber Pflanzenphysiologie, p. 618) has shown in a most 

 interesting manner how the essential structural relationships of the higher plants 

 are subservient to this chlorophyll-function. We can thus understand how the 

 leaf must have an entirely different shape from the root. The greater its surface 

 the greater the transpiration, and consequently the arrangement of the water- 

 conducting tissue in the leaf is especially adapted to carry out this function. 

 In order, however, that transpiration may not be so great as to act inimically to 

 the plant, we find numerous arrangements (to which we have already referred) 

 for retarding that process. 



If we turn now to the stem we find it to be the medium of communication 

 between the root and leaf ; it has to carry the substances absorbed by the root up 

 to the leaf and, conversely, to transport back again the materials manufactured 

 in the leaf. It has also to support the entire weight of the aerial part of the plant, 

 its own as well as that of the lateral organs ; and when we consider, not a small 

 annual herbaceous plant, e. g. Draba verna, but an oak-tree several hundred 

 years old, we can easily appreciate how rigid the stem must be. Each cell by 

 itself has a certain rigidity owing to the tension of its membrane, due to the 

 osmotic activity of the cell -sap. This alters, however, with the amount of water 

 present, and on hot summer days the rigidity due to turgescence is rapidly re- 

 duced by increased transpiration. All land plants of large size have, therefore, 

 as well a special mechanical tissue system, the thick-walled sclerenchyma. We 

 owe to SCHWENDENER (1879) the demonstration of the fact that this sclerotic 

 tissue is arranged in accordance with engineering principles, so that the greatest 

 effect is attained with the minimum expenditure of material. Sclerotic tissue is 

 also present in the leaves and roots, but it is distributed differently in these 

 situations, since the mechanical requirements of these organs differ from those 

 of stems. 



In this sketch we have limited ourselves to the vegetative organs and 

 have referred only to some of the most characteristic features exhibited by 

 these organs. A detailed description of the anatomical adaptations seen in 

 plants will be found in HABERLANDT'S (1896) Physiologische Pflanzenanatomie, 

 2nd ed., Leipzig ; to give such an exposition here would be foreign to the purpose 

 of these lectures. The general result, however, of our review is to lead us to 

 the conclusion that the structure of a member is adapted completely to the 

 function it has to perform. Doubtless, also, the cell of Basidiobolus is adapted 

 to the performance of its functions, although on account of the fact that all 

 functions are in that case carried out by one cell, the finer organs and their 

 adaptations are microscopic and cannot be distinguished in detail. It is not 

 to be wondered at that we do not know whether an elaborate differentiation, 

 such as occurs in the body of the higher plant, or such a purposeful division of 

 labour, occurs here also, or whether the single cell taken as a whole is able to 

 act like the complex apparatus of a higher plant, made up of millions of cells. 



Division of labour in differentiated plants has a far-reaching significance, 

 on which a few remarks must be made here. Whether the two cells resulting 

 from the division of the original Basidiobolus remains united or not is quite 

 immaterial, but it is quite a different matter in the case of the flowering plant. 

 Here the individual parts, whether they be the members visible to the naked 

 eye or the cells seen only under the microscope, are incapable of living separately. 

 A leaf, for example, torn off by a gale of wind rapidly dies ; it can manufacture 

 organic substance, it is true, but it withers for want of water. A root also can 

 take water and salts from the soil after the shoot has been cut off, but it soon 

 ceases to grow, because no organic materials are supplied to it. An isolated 

 sclerenchymatous fibre or trachea removed from the organism is a dead and 

 useless structure. Only when these units are bound together into a concrete 



