DESCRIPTIVE BOTANY. PART I. 



nutriment on which the young plant subsists until the 

 root is sufficiently developed to support it. But there 

 are other cases, as in the seeds of the castor-oil plant 

 (Rii-inus communis), the marvel of Peru (Miral>ilin 

 Jalapa), &c., where the cotyledons are thinner and 

 more leaf-like (fig. 23. a), and the embryo is wholly 



23 



or partially imbedded in a nutritive matter termed the 

 " albumen " (fc), which serves to develope the plant in 

 the early stages of its growth. The few exceptions 

 which occur in the dicotyledonous character of the 

 embryos of this class, will be noticed when we enter 

 into further details concerning seeds in general. 



(2.) Organisation of the Stem. 



The most important characters in the organisation 

 of most stems of this class, depend upon the manner in 

 which they increase in thickness. In young and suc- 

 culent stems, we find a solid cylindric or prismatic 

 mass composed of celhJar tissue, and termed the 

 " pith :" this is surrounded by a ring of vessels, consisting 

 of trachea? and ducts, and named the "medullary sheath." 

 The whole is coated by the epidermis. Afterwards, 

 a further development both of cellular and vascular 

 tissues takes place between the medullary sheath and 

 epidermis, and these form one layer of wood, and also 

 one layer of bark, by the time that a stem of one year's 

 growth is completed. During the second year, a fresh 

 development takes place between the wood and bark 

 previously formed. This fresh matter appears at first 

 as a semifluid or viscous mass, termed "cambium," 



