ELEMENTARY STIirCTrBE. 61 



Holding a sort of intermediate position between the internal 

 and external glands as above described, are the true nectaries of 

 flowers, which being strictly of a glandular nature will be most 

 properly alluded to here under the name of nectariferous glands. 

 They are well seen at the base of the petals of the common But- 

 tercup {fig. 148) and Crown Imperial {fig. 149). These glands 

 consist of a depression into which a honey-like fluid or nectar 

 is secreted by the surrounding cells. The tissue of the stigma 

 of Flowering Plants is also covered by a viscid secretion at 

 certain periods, and may be considered therefore as of a 

 glandular nature. 



On the young bark of most plants may be observed little brown, 

 generally oval projections, which have been called lenticels, or 

 lenticular glands from their supposed glandular nature {fig. 

 150 I, I). They have, however, no analogy with glands, but 

 are merely prolongations externally of the cellular envelope of 

 the bark. Their use is altogether unknown, although various 

 functions were ascribed to them at different periods before their 

 structure was properly understood. Adventitious roots, however, 

 come off from these points when a branch of a willow, upon 

 which they are very numerous, is placed in moist soil. 



6. — Intercexltilar System. — Having now described the dif- 

 ferent varieties of cells, and the modifications which they undergo 

 when combined so as to form the tissues, we have in the next 

 place to allude to certain cavities, &c., which are placed between 

 their sides. These constitute the intercelhdar system. 



Intercellidar Cajiah and Intercellular Spaces. — The cells being, 

 in the greater majority of cases, boundt d by rounded surfaces, or 

 more or less irregidar outlines, it must necessarily happen that 

 when they come in contact they can only touch at certain points, 

 by which interspaces will be left between them, the size of which 

 will vary, according to the greater or less roundness or irregu- 

 larity of their surfaces. When such spaces exist as small angular 

 canals running roimd the edges of the cells and freely communi- 

 cating with each other, as is especially evident in round or 

 elliptical parenchyma (j?^. 17), they are called intercelhdar passages 

 or canals ; but when they are of large size, as in spongiform 

 tissue, intercelhdar spaces {figs. 67 and 96, c). In most cases 

 these spaces and canals are filled with air, and when they occur 

 in any organ exposed to the atmosphere which possesses stomata, 

 they always communicate with them {fig. 107), by which means 

 a free passage is kept up between the atmosphere and the air 

 they themselves contain. The laticiferous vessels, as we have 

 already seen, appear to be formed out of the intercellular canals. 



Air Cavities. — In water plants the intercellular spaces are 

 commonly of largo size, and bounded by a number of small cells 

 regularly arranged, by which they are prevented from communi- 

 cating with each other or with the external air {fig. 151) : they 



