CHONDRUS. 459 



Laurencia pinnatifida begins at about 6 r above 0, and is 

 closely followed by Laminaria digitata, 5 ; above 0. Lami- 

 naria saccharina begins a few feet lower down. Sacchorhiza 

 bulbosa and Alaria esculenta still accompanied by Lami- 

 naria saccharina and digitata, the latter having about 

 reached its lower limit, are then met with at about 

 3' below 0. Ilalidrys siliquosa is found at a still 

 greater depth. 



These are the chief plants met with in descending from 

 the highest to the lowest water-marks. 



The data mentioned so far refer to plants which lie 

 exposed on the surface of the rock when the tide recedes. 

 It is important to mention this, as many plants rise to a 

 greater height when growing in pools. 



Laminaria digitata may rise to 9' above 0, and probably 

 higher still in a pool. Exposed, however, its upper limit 

 appears to be 4 ; lower. The plants at the former heights 

 are much smaller than those growing exposed lower 

 down. 



We can say that alga? exposed when the tide recedes 

 attain their best development in size and reproductive 

 powers in the lower part of the region to which they 

 belong. As they rise to the upper limits they become 

 smaller. They may, however, be found above their 

 normal limit in pools. The higher pool plants are always 

 smaller than the lower exposed ones. 



Chondrus crispus, as a plant lying quite exposed when 

 the tide recedes, extends from 3' to M above downwards. 

 It has been actually observed to about 3 r below 0. As a 

 general rule the upper plants are shorter, broader and 

 thinner (PL L, tigs. 4, 5), than the lower ones. The latter are 

 stouter, very much longer, and the frond is divided into 

 narrower lobes than are found higher up. When growing 

 in pools Chondrus crispus has been found up to a height 



