84 ON GUANO. 



renders it" utterly impossible to specify within the limits of this 

 article the soil proper for each particular species ; however, I think 

 it may be advanced as a rule not subject to many objections that the 

 whole of each genus are generally fond of the same compost. I 

 shall draw up a table of genera, of which any of the species are 

 known to require the aid of the greenhouse or stove ; showing that 

 peculiar soil most suitable to each particular genus ; deduced from 

 observations on the extensive collections I have had under my own 

 particular care, combined with those which I have had an opportunity 

 of making on others, as well in the vicinity of London as around 

 Dublin. 



The necessity of this combination is evident from the difficulty of 

 finding the whole of the genera here enumerated in any single col- 

 lection in the United Kingdom. 



ARTICLE VI. 



ON GUANO. 

 (Continued from page 65.J 



" The nectariferous juices, or, as they are commonly called, the 

 honey in flowers, are usually separated or secreted, by glandular 

 bodies called nectaries, and this honey has by many been supposed 

 indispensable in the fecundation of the seed ; but there are also 

 glands on the leaves and leaf-stalks (petioles) of many plants, which 

 perform the same office of secreting honey ; here, of course, it cannot 

 be of use for this purpose. Such glands exist on the petioles or leaf- 

 stalks of most of the acacia tribe; on the tips of three or four of 

 the lower serratures on the leaves of Grewia, on various parts of the 

 leaves or stems of the Balsam, on Passiflora, and many other plants. 

 These glands only secrete honey during the youth and growth of the 

 leaf; it is then only that their operation and beautiful structure can 

 be properly observed. When the leaf has attained its full growth and 

 perfection, the active part of these glands dries up, the time for ob- 

 serving their powers is past, and the leaf then proceeds in its own 

 important functions of elaborating the sap. It has been lately sur- 

 mised, and it appears to me with every probability of truth, that this 

 honey is an excretion of the superabundant and useless part of the 

 juices thrown off, after the leaf or flower has selected all that is 



