PHYSIOLOGICAL BOTANY, 225 



above and below them. The latter cells are large and 

 polygonal, tolerably uniform in diameter, and contain 

 large granules of starch. If we imagine to ourselves 

 these transverse cells extended longitudinally, they would 

 assume the same form as the polygonal cells existing 

 above them. These transverse cells appear to me to be 

 those last formed, and to have sprang up where the large 

 polygonal cells are separated from each other, and have 

 left gaps. That such gaps must be formed during the 

 growth of the parts is unavoidable. As the stem in- 

 creases in thickness, the woody bundles of the liber 

 become separated from the pith, and the layers of woody 

 tissue grow up between them. These certainly could 

 not force asunder the parts between which they grow, 

 but these latter must separate from each other, by virtue 

 of a distinct and peculiar vital power of expansion, to 

 allow the growth to take place. Physiologists, in giving 

 their attention to matters of little consequence, often 

 overlook others which are of great importance, as has 

 happened in the case of this peculiar property. Mohl 

 has nowhere shown, as he very modestly confesses, that 

 the increase of the cells is produced by the primordial 

 utricles. 



In other general respects, the observations upon this 

 utricle, detailed by Mohl, are, as might be expected, 

 minute and accurate. I have not only examined plants 

 which have been long kept in spirits, but also, and much 

 more frequently, such as have been macerated for some 

 time in nitric acid. It is unnecessary to neutralize the 

 nitric acid with carbonate of ammonia, and the section 

 only requires to be washed with water, to obtain the 

 result just as distinctly. Colouring the object with 

 iodine renders it still more distinct, and is, therefore, very 

 important. It is also unnecessary to select parts which 

 have not completed their growth ; all that is requisite to 

 obtain the same result is, that they should not have be- 

 come too hard and dry. I have tried this with many 

 plants ; but among them I shall only mention the leaves 



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