92 BOTANY 



the restriction of growth mainly to certain regions of the 

 surface, which became what we now call growing points ; 

 here the multiplication of cells led to the formation of 

 conical outgrowths, and these in turn were soon recognis- 

 able as branches. The larger the plant became the 

 more necessary was it for it to become branched in view 

 of the dangers from storms, and currents in the water, 

 as it would oppose much less resistance to the move- 

 ments of the stream or tide. 



The problem of the passage of the absorbed water, as 

 it made its way from exterior to interior cells, involved 

 the question of its being able to pass through the cell 

 walls that lay in its way. As the length of the body of 

 the plant increased with the putting out of branches, the 

 number of the cell walls became an inconvenient obstacle 

 to its passage. The flow being in a particular direction 

 caused the cells to stretch a little accordingly and so to 

 make them become a little longer than broad. This 

 elongation soon became advantageous, as with cells of 

 that shape there were fewer walls to pass in a given 

 distance. So gradually in the regions of transport 

 elongated cells became most usual. After a time the 

 end walls became perforated and later still dissolved 

 altogether, so that the water could pass easily along a 

 tract which was originally a number of columns of cells. 

 We can observe this change of internal structure taking 

 place in many plants to-day. Each column when it 

 has lost its end walls forms a vessel. We find them 

 fully developed only in terrestrial plants, but some of 

 the larger seaweeds contain structures much like them, 

 the end walls being perforated very freely. 



During the time these changes were taking place 

 in the arrangements of the interior, other signs of 

 specialisation of exterior parts made themselves visible. 

 With increase of size and great flexibility of body as in 

 the filamentous forms it became advantageous to be 



