PRESIDENT'S ADDRESS. liii. 



independently. Many of these simple cellular plants have their 

 cell-walls strengthened by siliceous matter, as is the case with 

 various forms of Dialomacece and Desmidece, whose propagation 

 appears to be carried out in two ways, in one, the original struc- 

 ture is repeated by budding, and in the other, by conjugation. 



The distinctive characteristics of the Phanerogamia, or flower- 

 ing plants, do not necessarily consist in bearing flowers, for they 

 may be reduced to a condition scarely distinguishable from the 

 fructification of the Cryptogamia, such as the Coniferae in which 

 the floral envelopes are absent. The growth of each plant is 

 accomplished by a continued extension of the axis, which from 

 time to time puts forth leaves at the internodes. Annuals sur- 

 vive one year only, biennials two, perennials shed their leaves and 

 flowers at the end of every season, while the axis preserves its 

 vitality. The venation of the leaves of Monocotyledonous plants 

 is parallel, that of Dicotyledonous (which have two cotyledons) 

 shows a higher development of the axis. The stem of a Dicotyle- 

 don, if cut across, shows a distinct division between the pith, 

 wood, and bark, the first forms the centre ; the wood is 

 intermediate between the two. The pith is surrounded by a 

 membrane, termed the medullary sheath. In the course of a 

 year or more the woody structure has a more or less distinct 

 appearance of division into concentric rings, varying in thickness. 

 The number of the layers is usually considered to correspond 

 with the number of years during which the stem has been grow- 

 ing ; this is not strictly true, it would be more correct to say that 

 each layer indicates an epoch of vegetation, which is in temperate 

 climates, usually, but not invariably, a year. In tropical climates 

 the epoch is much less. The rings are seen in the transverse 

 sections, traversed by lines radiating from the centre to the 

 circumference ; these are the medullary rays, which connect the 

 cellular tissue of the pith with that of the bark, and consist of three 

 layers ; that nearest the medullary rays is termed the liber, and 

 is separated from the wood by the cambium, a layer made up of 

 very delicate cells from which the new growth originates. It 

 is remarkable that in their early developed state, the stems of 



