14 ELEMENTS OF PLANT ANATOMY. 
In these few instances it has been discovered that the sepa- 
rate lamellae are added, as such, to the primary wall. Each 
layer is formed in some way out of the protoplasm and joined 
to the old wall. Subsequent growth in thickness of each or 
any layer may take place, so that this furnishes no evidence in 
FIG. 7. 
A Fragments of tracheids of the fir in tangential long section. t bordered pores. s striation 
of wall. #8 cell from the parenchyma of Dahlia tuber showing striation. x 240. 
(Accd. to Strasburger.) 
regard to the question of growth by intussusception or apposi- 
tion. Both of these methods of growth are possible here as in 
most other cases which have been studied. There is. much 
difference of opinion in regard to these two theories, though it 
is now generally believed that both methods of growth occur. 
The structure of wall described as striation is not limited to 
the above-mentioned instances, but is often found in cells of 
various kinds. Neither is it always connected with stratifica- 
tion, being often found in walls which show no appearance of 
division into layers. 
It has already been stated that there are two classes of so- 
called thick-walled cells, those designed principally for support 
and those both for conduction and support. In the former class 
