16 THE TRANSVERSE SECTIONS OF PETIOLES OF EUCALYPTS 



vent diffusion, a compromise has to be effected. The Wcalls are thickened here and 

 there, and diffusion occurs in the thin places. 



The sieve tubes, with their sieve plates, allow^ of the transport of indiffusible 

 substances, such as albuminoids. Just as the blood in our blood vessels is carried 

 bodily along, so is there a direct bodily transport of these indiffusible substances 

 through the meshes of the sieve. 



The next step is to vessels proper in the wood tissue, where there is no longer 

 even a sieve plate, but the transverse partitions are removed completely. In that 

 case the walls want special strengthening to prevent collapse, and so there are all the 

 admirable contrivances of annular and spiral vessels. The vessels technically belong 

 to the wood, but they are circulating tissue. They are for quick transport, the 

 quickest of all ; the cambiform cells the slowest, working by the slow process of 

 diffusion ; and the sieve tubes intermediate. The fibrous cells, or wood fibres, 

 especially the younger, have probably the task of conveying water, for they readily 

 take it in and readily give it off, so that the F. V. B., with its varied cells, resolves 

 itself into a contrivance for mechanical support on the one hand, and a means of 

 circulation on the other. 



To show the importance of the soft bast, it may finally be mentioned that it is 

 probably there albuminoids are manufactured, as well as conveyed to their destina- 

 tion. It contains more albuminoid matter than other parts, and it is thickly 

 bestrewn with crystals of oxalate of lime. The albuminoid matter thus produced is 

 not living protoplasm, but it is a necessary preliminary to it. The exact composition 

 of this raw material of life occurring in the soft bast is not known. It is quite 

 possible, however, that any living cell favourably situated may manufacture living 

 protoplasm when the proper material is supplied to it, for the final touch is the 

 touch of vitality to convert the lifeless mass into the living protoplasm. 



Before glancing at the other transverse sections in a comparative way, the 

 effects of the application of Schulze's solution on the different constituents of the 

 transverse section of E. globulus may be briefly summarised. The cuticle of the 

 epidermis is generally stained yellow and orange ; the cuticularised layer of a ruby- 

 red ; the cortical portion purple ; the lignified cells of the hard bast, orange ; the 

 soft bast, purple; and the woody tissue, orange, with a tint of brown. The contents 

 of the cortical cavities seem to be unaffected, and the crystals are dissolved. 



In the other transverse sections, the epidermis is generally a well-defined layer of 

 cells, with more or less thickened outer walls. In some this thickness is excessive, 

 specially in E. tetraptera, E. obcordata, E. pachypoda, and E. uncinata. It is worthy 

 of remark that these four are shrubs, or at most small trees, that the two former are 

 confined to Western Australia, while the two latter belong to desert country, and 

 constitute a portion of tbe Mallee scrub. 



There is a kino-like exudation often met with in some, such as leucoxylon, 

 lehrnanni, occidentalis, and alpina, being quite a feature in the last form. The 



