Vegetable Physiology. 



379 



in passing that a similar condition exists in the cellular struc- 

 ture of the cartilaginous Sea-weeds ; for instance in the caragheen 



Fig 2. 



A. Section of the cotyledon of a -n-hite haricot bean, showing a, the soft thickened cell-walls, which 



are coloured light-blue by solution of iodine ; b, the protoplasmic or albuminous substance in 

 the cells, coloured yellow by iodine ; and c, the starch-granules, coloured deep blue. Magnified 

 200 diameters. 



B, Loose starch-gi-anules, the larger having the striae, with a central cavity sending out radiating 



cracks. Magnified 400 diameters. 



or ' Irish moss,' and in Lichens, where the thickening of the walls 

 of the constituent cells converts the whole into a soft horny sub- 

 stance, in which the cavities of the cells occupy comparatively 

 little space. The thickening layers of these tissues are, how- 

 ever, distinguished by their peculiar consistence ; they appear 

 to be formed of a modification of cellulose approaching to starch, 

 for not only are they readily softened and dissolved by acids, and 

 sometimes capable even of assuming a blue colour with iodine^ 

 like starch, but on the recommencement of vegetation (germina- 

 tion of the seeds) they are, like the starch of wheat or the 

 potato, attacked by the nitrogenous formative layer, which had 

 previously formed them, re-dissolved, and more or less com- 

 pletely removed and conveyed to the developing regions of the 

 young plant. 



Structurally related to the foregoing, although very differen 

 in their physiological relations^ are the cells of the tissue called 

 epidermis. So soon as any rudimentary organ of one of the 

 higher plants has attained a distinct form, the layer of cells 

 which form its boundaries, those constituting its entire surface, 

 come into close union at their sides, and, assuming a special 

 mode of increase and expansion, present themselves in the perfect 



