514 PHYSIOLOGY. 



the pore, and either tears through or causes the absorption of the primary 

 membrane of the vessel. 



The Systems are combinations of tissues, of like or different 

 form and character, into elementary structures formed on definite 

 plans, and destined for particular purposes in the economy of the 

 plant. 



In the simpler plants there generally exists no distinction of 

 systems ; but even in the higher AlgsB and Lichens there is a dif- 

 ference in the cortical and medullary portions of the thallus. In 

 plants possessing stems and leaves, the nbro-vascular or wood- 

 system makes its appearance ; and we may distinguish in the 

 Phanerogamia three primary systems, viz. the Cellular, the Mbro- 

 vascular, and Cortical Systems. These are all formed of proper 

 constituent cells or tissues. 



Besides these, we have systems which are formed for the most 

 part by the interspaces between the cells of the above tissues, viz. 

 the Aerial System, consisting of intercellular passages, spaces, or 

 even large cavities ; and the Secretory System, including the milk- 

 vessels, reservoirs for secretion, glands, &c. 



The Cellular System. This name is applied to the cellular 

 tissues forming the great mass of the living structure of plants. 

 In the Thallophytes it forms the whole organization, the super- 

 ficial layer of the larger kinds of thallus not being a true cortical 

 layer like that of the higher plants. In the Mosses and Hepaticse 

 little is added to the cellular system, the fibro -vascular system 

 appearing in a very simple form in the stems, and the cortical in 

 the shape of an epidermis to the seta. In the higher Cryptogamia 

 and the Phanerogamia the cellular system is less predominant, 

 except in the temporary organs. In the stems and roots it forms 

 the pith and medullary rays of Dicotyledons, and the diffused 

 medullary system of Monocotyledons, together with the cambial 

 structures in all growing regions ; and it forms the mass of the 

 leaves and the parts of the flower. It is in this system that the 

 vital processes of vegetation are chiefly carried on. If the con- 

 stituent cells do not grow or divide, they constitute a permanent 

 tissue ; but if the cells divide, they form meristern, or generating 

 tissue. 



Meristem. That part of the parenchyma, or cellular tissue, 

 whose constituent cells multiply, or are capable of multiplying, by 

 division, as subsequently explained, is called meristem. According 

 to Hanstein's researches on the development of the tissues in the 

 embryo plant, the meristem or growing cellular tissue, in the course 

 of its development, gradually undergoes changes which result in the 

 formation of distinct layers of cells differing in size, form, and mode 





