§3 



mineral salts, more specially a quantity of calcic carbonate, or 

 chalk. As the water evaporates and the carbon dioxide gas 

 comes off, the chalk is precipitated, and that which is deposited 

 in the neighbourhood of the pit tends to aggregate around the 

 hairs, and becomes thus firmly held and prevented from falling 

 into the pit and stopping up the open water-pore, thereby 

 rendering it useless. In this manner the whole margin of the 

 upper surface of the leaf becomes covered with little masses of 

 chalk, indicating the openings through which the water is ex- 

 creted. In spite of this special provision, however, the older 

 glands frequently become inefficient on account of being choked, 

 the pit becoming completely filled with the very large concre- 

 tions of chalk formed, which appear to spread from the pit, 

 covering the entire lobe and even extending over other parts of 

 the leaf as well, in the form of a white incrustation. The 

 whole of the phenomenon may be easily seen by placing a 

 bell-jar over a vigorous plant. Since the air becomes saturated 

 with aqueous vapour, transpiration is reduced, and large drops 

 of water are secreted. On removing the jar the water rapidly 

 evaporates, and a deposit of chalk is formed. Such is the 

 mechanism of the water gland and pores in Saxifraga crustata, 

 which is unique as regards the special provision for the de- 

 position of the calcic carbonate, the distinct differentiation of 

 the gland tissue, the well-marked sheath for the gland of 

 endodermis, the extreme granularity of the protoplasm, and the 

 activity of function. In the water glands of the Crassulas, eg., 

 C. arborescens, which are the next most highly developed, we 

 have neither hairs, nor endodermis forming a gland sheath. 

 The protoplasm is not nearly so granular, nor the activity so 

 great. The diminution of gland tissue is often accompanied by 

 an increase in fibro-vascular tissue, which appears to replace it. 

 Thus, in the Crassulas, the reticulated cells at the base of the 

 gland are much more numerous than they are in Saxifraga, 

 and this seems to be a general tendency in the less highly 

 differentiated glands. Between the water glands of Saxifraga 

 and Crassula and those occurring in other plants there seems 

 to be a wide hiatus. The well-differentiated gland and con- 



