32 ANATOMICAL STRUCTURE OF THE LEAVES. 



variegated in the manner first described, continue to produce coloured 

 foliage year after year without manifesting any special signs of debility 

 or decay ; bnt their rate of growth is always slower than that of the 

 normal forms. Plants partially variegated often show signs of disease 

 in the coloured parts, which turn brown and die, in some cases 

 within a few weeks after it is produced, especially if the plant is exposed 

 to the direct action of the sun's rays. Partial variegation in vigorous-growing 

 kinds often disappears entirely in the course of a few years ; it is also 

 greatly influenced by' the soil in which the plants are growing, being 

 heightened in some situations or soon becoming obliterated in others. 



Glaucescence is quite distinct from variegation ; it makes its appear- 

 ance indifferently in young and old plants. It is always present in 

 the foliage of many species, in some of which it becomes greatly 

 heightened by age ; it also frequently appears with great intensity in 

 the young plants of species that are normally quite green or show it 

 but very faintly. The effect of glaucescence, as regards the aspect of 

 the trees, is to give them a greyish silvery hue, particularly pleasing 

 and beautiful in many plants belonging to the Cypress tribe and to 

 the Firs ; while it imparts a venerable hoary appearance to aged 

 Pines and especially to the Cedar of Lebanon. It is due in one form 

 to the stomata of the leaves, and it is not improbably an optical 

 effect arising from their close proximity and formal arrangement, 

 especially in the case of the white lines seen on the under surfaces of 

 the leaves of the Silver and other Firs, and in the leaves of Pines, 

 Junipers, etc. In another form it is caused by a resinous secretion 

 which is easily rubbed off by the finger, leaving the leaf quite green. 

 The minute structure of the leaves of Conifers afford a most 

 interesting study for those desirous cf gaining an adequate conception 

 of the apparatus by which the physiological functions of nutrition, 

 respiration, etc., are carried on, but a satisfactory treatment of the 

 subject would far exceed the limits of this work. The anatomical 

 structure of the leaves as seen in transverse sections of them, or 

 the most salient points of structure have of late years been so 

 frequently given with the descriptions of species, chiefly of the 

 AMetinecV, that the subject must not be entirely passed over. Much 

 stress too has occasionally been placed on certain anatomical characters 

 in the leaves as marks of generic differences, such as the position and 

 number of the resin-canals, which on account of their constancy in 

 the different genera into which the trees known under the general 

 name of Firs are now distributed, are frequently mentioned in 

 connection with them. But the similarity of the structure of the 

 leaves of species included in the same genus together with some 

 variability that has been observed in a single species does not admit 

 of much reliance being placed upon the anatomical characters as a 

 means of distinguishing species. The illustrations here given have 

 been especially prepared for this work by Mr. N. E. Brown, of the 

 Kew Herbarium, and are intended to convey a general idea only of 

 leaf structure of the Abietine?e. 



