STRUCTURE.] ORIGIN OF TISSUES. 23 



Natural History, vol. vi. p. 36. He is of opinion that the 

 cells of plants, including the so-called vessels, but excluding 

 laticiferous tissue, allow of two periods being distinguished 

 in their life.* In the first, which is that of their origin and 

 isolated independent development, the membrane forming 

 them grows, in its entire substance, by true intussusception. 

 But as soon as the cells have adhered, to form the cellular 

 tissue, and constitute the mass of a certain plant or its parts, 

 this mode of growth either ceases entirely, or recedes so far 

 into the background, that, from my observations up to the 

 present time, I cannot venture to maintain its continuance ; 

 but neither can I deny it, because of the frequently very 

 considerable expansion of the cells after the appearance of 

 the succeeding formations. But in every case a new and 

 very important momentum is added, by the deposit of a new 

 layer on the inner surface of the cellular wall, and indeed 

 everywhere, in the form of one or more spiral closely wound 

 bands *, so that the coils, without continuity inter se, still 

 mostly exhibit the completest contiguity. Hence proceed 

 all the varied formations of cells and vascular walls according 

 to the different influence of the following momenta : 



A. The most essential circumstance, and upon which is 

 founded the division of all these textures into two large 

 groups, that of Spiroids, and of porous formations, is the fol- 

 lowing : Either the cell has, at the time when the thickening 

 of its wall by spiral deposition commences, already attained 

 its complete expansion, or it has not. 



1. Let us, in the first place, consider the latter case. 

 Here, then, a second momentum becomes of importance; 

 it is the cohesion both of the fibre and the cellular wall, and 

 of the coils of the fibre inter se ; at the same time, therefore, 

 the number of fibres is likewise of value. 



a. Simple fibre. The cell still expands considerably from 

 the instant of its origin ; some convolutions cohere early, 

 others tear asunder : thus come annular vessels. In 

 this case the fibre is generally not united at all with the 

 cell membrane, or but loosely. 



* Thus it appears that Schleiden supposes all the secondary layers of tissue 

 to be spiral. 



