2 COMMON ORIGIN OF TISSUE. [BOOK i. 



developed in bodies that originally consisted of nothing but 

 cellular tissue ; an embryo, for instance, is an aggregation of 

 cells only ; after its vital principle has been excited, and it 

 has begun to grow, woody tissue and vessels are generated in 

 abundance. We must, therefore, either admit that all forms 

 of tissue are developed from the simple cell, and are conse- 

 quently modifications of it; or we must suppose, what we 

 have no right to assume, that plants have a power of spon- 

 taneously generating woody, vascular, and laticiferous tissue 

 in the midst of the cellular. M irbel has reduced the first of 

 these suppositions to very nearly a demonstration ; in a most 

 admirable memoir on the development of Marchantia he speaks 

 to the following effect : " I at first found nothing but a mass 

 of tissue composed of bladders filled with little green balls. 

 Of these some grew into long slender tubes, pointed at each 

 end, and unquestionably adhering by one of their ends to the 

 inside of the sac ; others from polygons passed to a spherical 

 form in rounding off their angles. As they grew older, other 

 very important changes took place in certain cells of the ordi- 

 nary structure, which had not previously undergone any 

 alteration : in each of these there appeared three or four 

 rings placed parallel with each other, adhering to the mem- 

 brane, from which they were distinguished by their opaque- 

 ness ; these were altogether analogous to annular ducts. The 

 cells become tubes did not at first differ from other cells in 

 any thing except their form ; their sides were uniform, thin, 

 colourless, and transparent ; but they soon began to thicken, 

 to lose their transparency, and to be marked all round from 

 end to end with two contiguous parallel streaks disposed 

 spirally. They then enlarged, and their streaks became slits, 

 which cut the sides of the tubes from end to end into two 

 threads, whose circumvolutions separated into the resem- 

 blance of a gun-worm." In these cases there can, I think, 

 be little doubt that the changes witnessed by Mirbel were 

 chiefly owing to the development of a spiral thread in the 

 inside of the tissue ; he, however, did not consider it in that 

 light. 



The best general view of this subject is that of Schleiden, 

 of which a full translation is published in the Annals of 



