THE VEGETABLE ORGANIZATION. 291 



sides of the generating cell, we can conceive that each of them 

 is nothing else than the development of one of the globules of 

 which the coats of the cell are composed."* 



It has been before observed that the development of the cells of 

 plants is materially influenced by the varying circumstances of 

 pressure under which it takes place. When the pressure is 

 unequally distributed, being less in one direction and at the same 

 time considerable in all others, the cells will be much modified 

 in their shape. They will be compressed according to the 

 direction in which the pressure is applied, and the active principle 

 of growth continuing to be exerted chiefly in the direction of 

 least pressure, a series of elongated cells will be formed, ap- 

 proaching more or less to the form of tubes. To this modifica- 

 tion of cellular structure, M. Link has given the name of 

 elongated tissue. Sometimes the elongated cells, instead of 

 being cylindrical or prismatic in their shape, are fusiform or 

 spindle-shaped, that is, narrower at the extremities than in the 

 centre. These are the clostres of M. Dutrochet. The medullary 

 rays or strios found in the stems of dicotyledonous plants are 

 also elongated cells, but placed in a horizontal position, instead 

 of a vertical one, which is the usual arrangement of the cellular 

 texture. 



Such is the elementary structure and development of the 

 cellular tissue of plants; we have now to direct our attention 

 to the various modifications of vessels constituting the vascular 

 tissue. " Vessels," says M. Richard, " are layers of elementary 

 cellular tissue, rolled up in such a way as to form canals, or 

 cells, which are more or less elongated and placed end to end, 

 and whose partitions have often disappeared."f This definition 

 of vessels, though given by one of the latest and best authorities, 

 is extremely vague and imperfect, and affords us little insight into 

 the gradual or progressive development of this partof the vegetable 

 organization. From what has been stated before respecting the 

 modifications of the cellular configuration induced by pressure, it 

 will appear upon consideration that there is a tendency, under 

 certain circumstances of pressure, to atransition from the cell to the 

 vessel or tube : the elongated cell, for instance, is in fact a short 

 tube, closed at both ends, and it is precisely under the circumstances 

 of pressure by which the elongated cell is produced that we find 

 the vascular structure to be developed. The lateral pressure in 

 vascular plants, arising from their external form, is very con- 

 siderable; whereas the pressure in the cellular plants is for the 

 most part much more equallj^ distributed. It does not seem 

 improbable, also, that the mode in which the cells are arranged 

 together may exert some influence upon the development of the 

 vascular structure. When arranged in close contact, there will 



* Raspail. Elements of Organic Chemistry, translated by Henderson, p. 2. 

 t Richard's Elements of Botany, by Clinton. Ed. 4. p. 15. 



