8o 



In the species of Saxifraga (such as S. crustata) they appear to 

 occur only on the upper side of the leaf, at the margins, 

 in connection with the little depressions or pits between the 

 lobes or crenations ; in Crassula lactea and C. coccinea, on the 

 upper and under margins, and in Sedum Sieboldii, only on the 

 under side. 



In Saxifraga crustata, which, according to the researches of 

 W. Gardiner* has the most highly differentiated gland at 

 present known, the adult structure of the gland is somewhat 

 as follows : — The peripheral termination or ending of each 

 of the fibro- vascular bundles which ramify through the me- 

 sophyll of the leaf presents the appearance of a swelling or 

 dilatation at its extremity, which is roughly pear-shaped in 

 outline, being broad towards the surface of the leaf, and taper- 

 ing inwards towards the fibro- vascular bundle. Each of these 

 dilatations is placed immediately under the bottom of one of 

 the depressions which exist between each of the lobes of the 

 leaf, and forms what is known as a water-gland. By far the 

 larger portion of the tissue of the gland is made up of poly- 

 gonal cells, slightly longer than broad, closely fitting one to the 

 other without intercellular spaces. These cells are derived 

 from the ground tissue at the apex of the termination of the 

 fibro-vascular bundle. The cell-walls are thin, and the cells 

 themselves are much smaller than those of the surrounding 

 ground tissue. They contain no chlorophyll, and their proto- 

 plasm is very granular. The gland is invested by a sheath of 

 cells containing chlorophyll corpuscles of one or at most two 

 layers thick, which entirely surround it, with the exception of 

 that portion of it which is covered at the surface of the leaf 

 with epidermis. This sheath is continuous with the endodermis 

 or bundle-sheath of the fibro-vascular bundles of the leaf, and 

 by means of it the very definite group of cells which form the 

 gland are exceedingly well marked off from the ordinary ground 

 tissue which surrounds it. At the inner extremity of the 



* See an admirable paper by him entitled " The Development of Water Glands in 

 the leaf of Saxifraga Crustata," in the Quarterly Journal of Microscopical Science, New 

 Series, No. LXXXIII, July, 1881. 



