582 VISIBLE CONSTRUCTIVE ACTIVITY IN PROTOPLASM. 



accordingly besides the cells which are still formative and promote growth, many- 

 others are present which undergo no further changes, whose size and shape is 

 permanently retained, and which have therefore been termed permanent cells. 



Organically-connected groups of permanent cells are called permanent tissue in 

 opposition to the groups of constructive, dividing, and changing cells, the so-called 

 meristematic tissue. All permanent tissue is obviously produced from meriste- 

 matic tissue, and the meristem is ultimately derivable from a single cell capable of 

 division. 



The cells of meristems exhibit only very slight variations in form. It is 

 impossible to recognize what forms the permanent tissues produced from them will 

 in time assume. Of four exactly similar meristematic cells, the first may become 

 the starting-point of several flattened epidermal cells devoid of chlorophyll; the 

 second for the formation of a group of green palisade-cells; the third for a bundle 

 of elongated, thick- walled bast-cells; the fourth for several delicate- walled, large 

 parenchymatous cells. It is difficult to explain how this comes about, and we 

 relinquish the attempt to give a full explanation here. . Only this much may be 

 remarked, that whilst the stimulus to these metamorphoses comes from outside, and 

 external conditions have a determinative influence on the size of the developing 

 permanent tissue, the shape, outline, and definite configuration which the indi- 

 vidual cells in the permanent tissue assume, as well as the arrangement of the 

 various cells in space, are independent of external influences. Just as in a plant the 

 first division-walls assume a position defined beforehand in the dividing apical-cell, 

 the further metamorphoses of the daughter-cells proceed within the limits settled 

 by the specific constitution of the protoplasm, so the transformation of the cells of 

 the meristem into cells of permanent tissue is governed according to a plan of 

 construction peculiar to, and hereditary in, each species. 



This law, derived from numerous facts, of the independence to external 

 influences of the constructive plan and character of the cells, seems to be con- 

 tradicted by the fact that alteration in the outline of individual cells can be 

 produced by strain and pressure. Spherical cells with elastic, flexible walls may be 

 changed by strain into ellipsoids; in consequence of all-round pressure a spherical 

 cell may assume the form of a rhombic dodecahedron, or by lateral pressure, the 

 form of a six-sided prism. In explanation of these conditions it has been pointed 

 out that peas which are made to swell up in a cubical, thick-walled vessel, by 

 pouring water over them, assume the shape of rhombic dodecahedra, because each 

 individual pea is in this way allowed the greatest possible room together with the 

 utmost economy of space. We are again reminded of the fact that the structure of 

 slate-like stones is dependent upon the pressure acting upon the mass so far at least 

 that the planes of cleavage and stratification are always at right angles to the 

 direction of the pressure. But however valuable these facts are in the explanation 

 of the condition of the form of inorganic bodies, they are of little significance to the 

 question in hand. No one will deny that spherical cells on which an equal pressure 

 operates from all sides may assume the shape of dodecahedra, but this form is not 



