HuNRY AND Fi.oou — T/ic Douglas Firs. 87 



protective foliage in tlie Rocky Mountains and in southern California re- 

 spectively — regions notable for tiieir long, dry, and hot summers. 



3. Hypodenn. A layer of thick-walled liypodermal cells is practically 

 continuous all round the leaf in P. i/lanca, V. macrocarpa, and P. sLriensis. It 

 is present only in the central part of the leaf in P. Dowjlasii, P.japonica, and 

 P. Forrcstii. The continuous hypoderm seems to be a xerophytic character, 

 the three species in which it is present all living in dry regions. In P. Douglasii, 

 var. caesia, which is more xerophytic than the type, hypoderm is a little 

 developed elsewhere than in the centre of the leaf. 



4. Idiohlasts. These are peculiar stellate or irregularly radiate cells, which 

 ramify between the ordinary parenchymatous cells in the leaf. They are 

 hollow, with thick walls and naiTow lumina running up the arms of the star. 

 In 1876 M'Nab discovered idioblasis in the leaves of the Eocky Mountains 

 Douglas Fir, but could not find them in the Pacific Coast species. In the 

 present investigation M'Nab's observations have been confirmed; and the 

 idioblasts have been proved by various chemical tests to be formed of lignin. 

 They have been found to be most numerous in P. glauca, rather abundant in 

 P. japonica, P. sinensis, and P. Forrcstii; very few in P. macrocarpa and 

 P. Douglasii, var. caesia ; and totally absent in typical P. Dour/lasii. 



The significance of idioblasts is obscure. The term idioblasts was 

 originally applied by Sachs to individual cells strikingly different from their 

 neighbours ; and he named hard thick- walled idioblasts, such as those now 

 described, stone-cells or scleroblasts. Haberlaudt^ refers to these as astro- 

 sclereides. 



Idioblasts are not confined to Pseudotsuga, as they occur in other conifers 

 and in ordinary flowering plants. As to their function, several theories have 

 been brought forward. One theory is that they act as water reservoirs. This 

 is supported by the fact that they largely occur in xerophytic plants; but in 

 opposition to this it may be pointed out that the amount of water they could 

 store would be very small, and that the plant would have diiBculty in extract- 

 ing the water for use. Sachs, indeed, says that they have such thick walls 

 that their contents are of little physiological importance. 



Another view, supported by iJe Bary,'^ Haberlandt, and Bower,' is 

 that idioblasts act as scaffolding to strengthen the leaf and keep it distended 

 and of a leathery consistence. It is difficulc to see, however, how they would 

 act as a skeleton, when for the most part they are embedded in the substance 

 of the leaf, are widely separated from one another, and do not extend to the 



' Physiological Plant Anatomy, 158(1914). 

 ■ Comparative Anatomy, 424 (lSS-1). 

 ^Botany of the Living Plant, 144 (1919). 



