SUPPLEMENT 89 



obviously of very great importance in the life of the plant. They render 

 possible a very general interchange of materials between cells, and they must 

 also be often the means of transmission of stimuli (Lecture XL). By their 

 means the plant body, which is broken up by its cell-walls into thousands of 

 individual compartments, once more becomes a unity. At least one is able 

 to understand very much better the unity of response so often exhibited by the 

 entire plant a response conditioned by the various responses of the individual 

 cells if a single protoplasmic body permeates the whole plant (KuHLA, 1900). 



Although many protoplasmic bodies penetrate through the unpitted wall, 

 still most of them will be found in groups in the pits, and we are led to conclude 

 that they are the cause of the correspondence of the pits. In addition to the 

 fine communicating threads which penetrate the closing pit membrane, we find 

 also coarser protoplasmic bridges which show themselves, e. g. in sieve tubes, 

 after the dissolution of the entire closing membrane in certain cells. The 

 difference between the two types is naturally a gradual one. One has, however, 

 been accustomed to regard the sieve tubes as fusions of individual cells placed 

 end to end ; one also assumes that the individuality of the cells is lost on account 

 of these coarser intercommunications. Cell fusions are found also in the forma- 

 tion of tracheae, where the communication between the individual segments 

 of the fused elements is much wider. 



Our discussion of protoplasmic bridges and cell fusions has gradually led 

 us from the consideration of the cell-wall to that of the cell contents, to whose 

 varied configuration is due the whole series of cell-forms functioning in different 

 ways. We may note only that in the organs which subserve water carriage, 

 namely, the vessels, the living contents entirely disappear, and are replaced by 

 water and air, whilst in typical cells sometimes it is protoplasm and its appa- 

 ratus, especially the chloroplasts, that is especially prominent, sometimes it is 

 the vacuole. There is no need for us at present to discuss these relationships in 

 detail. 



The cells now unite together into tissues, in such a way that, as a rule, long 

 stretches are composed of the same type of cell. In contrast to these there 

 are the so-called ' idioblasts ', which make their appearance as solitary units 

 among otherwise homogeneous tissues. Examples of these are often met with 

 in secretory receptacles, as isolated sclerotic elements, or finally as internal 

 hairs. It should be the task of developmental physiology to elucidate the 

 factors which induce the appearance of structures like these, and to determine 

 why a definite cell, which to all appearance could have developed otherwise, 

 should become an idio blast. The combination of cells into organs of higher 

 rank, which is obviously subject to certain laws, should also be capable of 

 causal explanation ; meanwhile, we can only say that the tissues, like the 

 externally visible arrangement of the lateral members, co-operate in producing 

 the symmetry of the whole body. 



We have learned how the external subdivision of parts and the internal 

 architecture of the complex plant come into being, so far as the main principles 

 are concerned. If we now glance at the function of these parts, we discover, 

 to begin with, that there is an intimate relation between function and structure. 



298, 1. 44, for 85 read 74 



299, 1. 21, after still read (40, CATTERINA, 1904) 



11. 29-33, for of 75 ... cardinal read of 70 (CATTERINA, 1904 ; MIEHE, 

 1907) ; Algae which grow in natural hot springs do not appear to be able to 

 stand temperatures as high as that (LpWENSTEiN, 1903). In general terms it 

 may be said that, in the case of plants inhabiting cold regions, the two extreme 

 cardinal 



11. 44, 49, for 91 read 80 



