ORGAXS Ot KUTKITIOX. I 3 



of Avliich Other spiral vessels are foraied, as well as annular, pitted, 

 and reticulated ones ; these are bound together and succeeded 

 by a number of delicate parenchymatous cells, and on the out- 

 side of these some laticiferous vessels, and at first a cambium 

 region which is gradually converted into thick-sided prosenchy- 

 matous cells resembling those of the liber of Exogens. In this 

 case the development of the vascular bundles, like those of Ex- 

 ogenous stems, is gradual, the inner part of each being first 

 formed, and growth proceeds progressively to the outside, hence 

 such are likewise progressive bundles, but, as such bundles have 

 no external layer of growing cells resembling cambium, no in- 

 crease in size takes place in them in successive seasons. Hence 

 the new vascular bundles are not developed in continuity with 

 the old, but remain distinct and of a limited size. Such vascu- 

 lar bundles are therefore named definite vascular bundles. 



In Acrogenous stems the vascular bundles are chiefly made 

 up of vessels of the scalariform, annular, or spiral type, accord- 

 ing to the different orders of Acrogens from Avhence they have 

 been derived ; these are suiTOunded by delicate tubular cells, 

 and the Avhole is enclosed by a firm layer of wood-cells. Such 

 bundles only grow by additions to their summits, and as the 

 elements of Mhich they are composed are not formed in succes- 

 sion like those of indefinite and definite vascular bimdles, but 

 simultaneously, they are called simultaneous vascular bundles. 



The distinctive appearances which we have thus seen to occur 

 in the stems of the three plants above noticed are also accom- 

 panied by certain ditferences in the structure of their embryo. 

 Thus plants with Exogenous stems have an embryo with two 

 cotyledons ; those with Endogenous stems have but one cotyle- 

 don in their embryo ; while those with Acrogenous stems have 

 no proper embryo, and consequently have no cotyledons. Hence 

 exogenous stems are also termed Dicotyledonous ; endogenous 

 stems Monocotyledon ous ; and acrogenous stems Acotyledonous. 

 Eor reasons which we shall describe hereafter, the latter terms 

 are in some cases to be preferred to the former. In the suc- 

 ceeding pages we shall use them indiscriminately. With these 

 general remarks on the structure of the three kinds of stems 

 we now proceed to describe them respectively in detail. 



A. Exogenous or DicoTYLEDOxors Stem. — Those plants 

 which possess exogenous stems are commonly termed Exogens, 

 and they constitute by far the largest number of plants in every 

 part of the globe. This kind of stem is alone found in the trees and 

 shrubs of this country, and all the colder regions of the earth. 

 In warmer cUmates it occurs associated wdth others possessing 

 endogenous and acrogenous stiiicture. 



In the embryo state, the exogenous stem is entirely composed 

 of parenchyma. But as soon as growth commences, some of 

 its parenchymatous cells become developed into vessels and wood- 



