12 CELLULAR TISSUE. 



above appearance, Figs. 2 and 5. Sometimes the vacant spaces 

 in the lining membrane form bands, Fig. 6 ; sometimes oval 

 spaces, Fig. V ; sometimes it takes a spiral ar- Fig 9 

 " rangement, Fig. 8 ; sometimes that of star- 

 shaped actinenchyma, as in Fig. 9, the pith of 

 the juncus. 



13. To cellular tissue has been assigned the 

 same place in the vegetable economy that flesh su^shaped 

 occupies in the animal, and we have no hes- celL 

 itation in yielding to it in every respect the importance this 

 comparison gives it. It constitutes the basis, physically con- 

 sidered, of the vegetable kingdom. 



14. Although the cells are without visible pores, yet the walls 

 are permeable to fluids, as is proved by their being sometimes 

 full, and at others empty. This may also be shown by taking 

 a piece of the pith of the elder and letting a part of it com- 

 municate with water, when the whole mass will become saturated 

 with it ; and it is a fact well understood at the present day, that 

 animal and vegetable membrane, even when not under the in- 

 fluence of vital power, is permeable to fluids under certain cir- 

 cumstances ; that is, when opposite sides of the membrane are 

 exposed to fluids of different density and the fluids are capable 

 of wetting the membrane. 



This passage of fluids through membrane was called, by Du- 

 trochet, Endosmosis (endon, inwards, and mao, I strive), and is 

 one of the most common agents used by nature in the vegeta- 

 ble economy for accomplishing her purposes. 



The reverse motion, that is, from within outwards, was called 

 Exosmosis (exo, outwardly). They both take place at once, but 

 the thinner fluid usually moves the most rapidly. 



15. The bursting of capsules is produced by this cause. One 

 may convince himself of the truth of this assertion by closely 

 inspecting the bursting of the common " Touch-me-not :" the 

 inner cells have become in maturity more firmly compressed and 

 smaller, while the outer ones are, in moist weather, turgid and 

 elastic, and by slight agitation, the cohesive resistance of the 

 valves is overcome, and a sudden bursting of the capsules is the 

 consequence ; the equilibrium of the pressure being produced 

 by the coiling up of the valves. The opening and shutting of 

 flowers at certain hours of the day, is undoubtedly another 

 effect of the same cause. 



13. What place has been assigned to the cellular tissue? — 14. How is it 

 proved that the walls of the cells are permeable to fluids ? What fact is now 

 well understood? What did Dutrochet call it ? — 15. What are some of the 

 effects of it? 



