. MOVEMENTS OF THE PARTS OF PLANTS. 547 



reason being assigned as to how this effect is brought about. It appears 

 to me that the three following points ought to be distinguished : 



1. Vegetable membrane is certainly only elastic in a slight degree ; it 

 may be distended as almost all other organic substances, but again 

 resumes its former volume on the withdrawal of the tension. The 

 parenchyma of the living plant, in consequence of endomosis, is con- 

 stantly in a state of tension ; each cell occupying a greater space than 

 belongs to it according to the natural circumference of its membrane. 

 On removing, however, a portion of the liquid thus distending it, the cell 

 contracts to its natural size. This effect, trifling as it may be in the 

 single cell, must yet become perceptible when hundreds of cells are taken 

 into consideration. Microscopical observation proves that this is really 

 the case. If we cut off the larger part of a succulent plant when it is 

 distended with fluid, as the joint of an Opuntia, or a large succulent 

 leaf, and allow it to remain for a short time in a dry place, the loss of 

 weight will prove to us that a part of the water has evaporated, and 

 exact measurements will prove that a simultaneous, but very slight, con- 

 traction to a smaller volume has taken place. Nevertheless, however, 

 we find all the cells entirely filled with juice, even on the most accurate 

 microscopical examination : and none exhibits in its membrane the 

 slightest fold, all appearing in a state of absolute tension. Simul- 

 taneously, therefore, with the evaporation of the water, there must have 

 taken place a slight contraction of all the cells. Let us apply this to the 

 external succulent layer of parenchyma in the fruit of the almond. 

 When distended by juice, 'the number of cells suffices perfectly to enclose 

 the hard stone, which is but little changed in volume by the process of 

 drying. But when the cells, becoming ripe, gradually lose their fluid ' 

 contents (which are no longer supplied by the fruit-stalk), a stretching 

 takes place by means of the contraction of the individual walls of cells 

 that are firmly connected with each other, the envelope becomes too narrow 

 for the stone, and if, as really occurs, there happens to be a layer of cell 

 tissue in which the cohesion is not so strong as the expanding power, 

 this layer is torn, and the cleft thus caused becomes wider the further 

 the evaporation of the water proceeds. 



2. To this condition we must add, as its continuation, a second and a 

 much more remarkable phenomenon. The thin membrane of the cell is 

 flexible in the highest degree, and on the liquid evaporating from the 

 cells without their being simultaneously filled with air, the cell dimi- 

 nishes in volume from the pressure of the external air, in the same way 

 that an animal bladder filled with water, gradually losing its water without 

 the vacant space being filled with air, cannot be distended to its former 

 volume without being torn. 



3. Vegetable membrane is very hygroscopical, and becomes distended 

 by moisture and contracted by dryness. But both take place in a very 

 different degree, according to two concurring circumstances. The more 

 the membrane approaches, in its chemical constitution, jelly, the more it 

 contracts when in the act of drying up ; and the more it approximates 

 to the nature of perfectly developed cellulose (membranenstoff), the 

 slighter is the expansion when exposed to the action of moisture. The 

 membrane, when of the same chemical nature throughout, appears to 

 contract the more the thinner it is, and the less the more it is thickened 

 by secondary deposits. This latter view agrees with the circumstance 

 that all spiral fibres (which, as we are in the habit of isolating them, con- 

 sist externally of the spirally-torn primary cell-membrane, and internally 



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