40 Transactions of tJic [Sess. 



nutritive organs is to be found among Acotyledons. This class 

 includes such organisms as the Algfe, Fungi, and Lichens, whose 

 nutritive organs consist of a single cell, or of an expansion of cellu- 

 lar tissue — the thallus ; but in the higher forms of Acotyledons, 

 such as Mosses, Ferns, Club-mosses, &c., structures resembling in 

 general form and function the root, stem, and leaf, are distinctly 

 marked, Mfhich thus approach the Monocotyledon and Dicotyledon 

 type. The greatest differences are seen vi^hen the structure of the 

 stem and leaf of each of these three classes is compared. The 

 structures themselves are composed of the same elements, but differ 

 in the mode of their arrangement. Eoughly, we distinguish in the 

 stem of a Dicotyledon, such as a Maple or Pine, three portions, — 

 an outer bark portion, an inner wood, and a central pith portion. 

 More strictly, however, the bark consists only of the outer layer of 

 cells, which have become corky, within which lies a green cellular 

 rind. Between this green cellular rind and the pith lie what are 

 termed the Fibro- Vascular Bundles, consisting for the most part of 

 wood and bast, the latter lying exterior to the wood. A transverse 

 section of the rhizome of a Fern exhibits an arrangement of these 

 bundles in the form of an irregular circle, while in that of a Cane 

 or Palm they are scattered irregularly in the abundant pith; and in 

 the Maple, Pine, or Rose they arrange themselves in collateral 

 wedges, so that the wood forms a circular band, which broadens 

 each year, enclosing a central pith, and coated externally by the 

 peripheral bark. As a rule, bark is only present in Dicotyledons. 

 Its absence is a characteristic mark of Monocotyledons. The vena- 

 tion of the frond of the Fern is forked ; in the Monocotyledon, par- 

 allel ; and in the Dicotyledon it is reticulated. Two other points of 

 distinction need only be mentioned, as they give rise to terms of 

 no uncommon occurrence. The first is that of the growing point 

 [puncturn vegetationis), and the consequent development of the stem; 

 and the other is derived from the character of the root. In certain 

 Acotyledons (not, however, universally) the growing point consists 

 of a single apical cell, from the repeated subdivisions of which 

 stem, roots, &c., originate, and hence they are spoken of as Acro- 

 gens; while Monocotyledons and Dicotyledons are distinguished as 

 Endogens and Exogens I'espectively. From the character of the 

 root, these three were again distinguished as Heterorhizal, Endo- 

 rhizal, and Exorhizal respectively. 



But the chief peculiarities of these three groups lie in their repro- 

 ductive organs. In Monocotyledons and Dicotyledons reproduction 

 is effected by what are termed Flowers, hence the name Phanero- 

 gams ajiplied to these two classes. In contradistinction to this, 

 Acotyledons, which have no such conspicuous flowers, have been 

 termed Cryptogams, a tei'm which refers to its hidden or secret 

 method of fertilisation. Dicotyledons are generally distinguished by 



