with special Reference to the Mode of Connexion of Cells. Ill 



observing them in adult lignified and suberised tissne, it seems 

 certain that they will be found there also. There can be little doubt 

 that they occur universally in the cells of all the tissues of all 

 plants. 



From this arises the fundamental conception that the plant body 

 must be regarded as a connected whole, and that the cell walls 

 occupy only a subservient position. Thus our views as to the ulti- 

 mate histology of tissue must be considerably modified. A new vista 

 also opens to cytological research in the direction of the accurate 

 determination of the distribution and orientation of the threads in the 

 various tissues, which can hardly fail to lead to important results. 



Should the structure presented by the external walls of the epi- 

 dermal cells of Tamus communis and LiUum martagon he found to be 

 of general occurrence, we shall be prepared for most interesting 

 results when the examination is extended to secreting gland cells : 

 to such non-cellular organisms as certain algse and fungi, and to such 

 unicellular bodies as spores and pollen grains. 



Two important functions are, doubtless, performed by the con- 

 necting threads,, viz., the conduction of impulses and the conduc- 

 tion of food. As to the first, there can be no question; and 

 as to the second, one cannot but reflect that it must be of the 

 greatest advantage to the plant to be able to transmit from cell to 

 cell as occasion requires, and in a definite and determinate direction, 

 highly organised food supplies and even protoplasm itself. It is, of 

 course, possible that in the threads themselves a definite division of 

 labour may occur as regards the transmission of food and the con- 

 duction of stimuli. 



The consequences which arise from our more perfect knowledge of 

 plaut histology are obvious and far reaching. 



In the first place we learn that the structure exhibited by sieve- 

 tubes, which in the past was regarded as peculiar, is shown to 

 be typical of cells generally, with this slight difference, however, 

 that in sieve-tubes a secondary enlargement of the pores appears to 

 occur. All the cells of a tissue must be regarded as being connected 

 together by delicate groups of protoplasmic threads, which traverse 

 either the general wall or the pit-closing membrane, just as the 

 sieve-tube threads traverse the so-called "sieve plate," a fact which 

 at length enables us to do tardy justice to the dominant position 

 occupied by the protoplasm of the plant body, and to understand 

 how the deep-seated cells of a tissue can telegraph their needs to 

 those at the periphery, cell after cell taking note of the wording 

 of the message, or how the peripheral cells may communicate to the 

 interior their sense of gravity, light, heat, or touch, to which the 

 whole organ may reply as its peculiar organisation directs. 



As an integral part of cell structure, the connecting threads con- 



