Comparative Physiology. 387 



attached to its wall or free in its cavity, sometimes permanent. 

 From the observations of animal physiologists, and j)articularly 

 of Schwann, the process of developement and growth of the 

 tissues of animals are exactly the same. Nearly all the tissues 

 have been shown to be formed from nucleated cells, previously 

 develoj)ed in a homogeneous formative mass. The order of 

 developement of the cell and its nucleus or cytoblast, and 

 secondary nuclei within this, as far as it has been observed, 

 appears also to be the same in animals as in jjlants. Some of 

 the tissues of animals, like the cellular tissue of plants, retain 

 the cellular form, while others, like the more highly developed 

 vegetable tissues, assume different forms." In the growth of 

 marchantia, Mirbel discovered that they increased by the pro- 

 duction of new cells alternately between every two of the old ; 

 new rows of cells are thus formed, and they extend laterally by 

 additions outside, as well as by superimposed rows. The woody 

 fibres pass downwards from the leaves through the cellular 

 tissue, leaving openings of cellular matter in exogenous plants 

 at the medullary rays, which connect the interior and exterior 

 of the stem. The woody and cellular matter forming the basis 

 of the vegetable structure are thus developed. While the tissue 

 is young and succulent it expands and stretches freely, according 

 to the heat and moisture supplied ; and, from the power of en- 

 dosmose, produced by the light and heat acting from above, 

 thickening the sap by evaporation and chemical action, and 

 attracting the thinner fluid upwards, the expansion is principally 

 in a longitudinal direction upwards, till the leaves are fully 

 formed and commence to solidify and ripen the branch, by the 

 elaborated sap sent downwards. Some plants make the whole 

 of their shoot at once in the early part of the season, others 

 have a spring and autumn growth, and some continue extending 

 the Avhole season round. 



Thus far observation has conduced to establish the theory of 

 developement. We are thus taught that to encovirage the soil, 

 the stomach of a plant, to perform its functions, the soil must be 

 properly pulverised in dry weather, as I have often elsewhere 

 repeated in other essays. Too little regard is had to the me- 

 chanical operation of the pulverisation of the soil, and taking 

 advantage of tids of weather ; and too little allowance is given 

 for unforeseen adverse occurrences of weather and soil, in the 

 experiments instituted in various quarters on manures. To prac-r 

 tical men, it is well known that the state of the soil has often 

 more effect on growth than the food deposited ; if the stomach 

 of an animal does not perform its functions properly, it will be 

 in vain to load it with food. The function of absorption is 

 increased by keeping the soil loose and porous, to enable the 

 roots to spread and ramify, which is also greatly encouraged by 



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