CHAP, i.] PLANT ARCHITECTURE. 21 



plant constitute the portion called wood. To under- 

 stand the value of fibro-vascular bundles to the plant, it 

 must be remembered that Algae, growing in water, are 

 not furnished with an epidermis, consequently every por- 

 tion of the plant's surface is capable of absorbing water 

 containing in solution the gases required for food and 

 respiration, also certain other elements of plant food held 

 in solution, hence no special structures for the conduc- 

 tion of water from one portion of the plant to another 

 are required. In land plants, on the other hand, the 

 leaves and other green parts developing in the air are 

 completely surrounded by a waterproof epidermis, that 

 prevents the cells from taking water from the air, and 

 no water enters the plant through the stomata, conse- 

 quently the quantity of water required by the above- 

 ground portions of the plant is in the first instance 

 obtained from the soil by the underground portion or 

 root, and thence passed up into the leaves. The primi- 

 tive unmodified tissue, known as fundamental tissue, of 

 which seaweeds are composed, does not possess the power 

 of transmitting large quantities of water from one part 

 of the plant to another, hence the necessity of a portion 

 of this tissue becoming modified into fibro-vascular 

 bundles in land plants, as these structures, especially the 

 thick-walled portion called wylem, are good conductors 

 of water, and thus serve as channels of communication 

 between the source of water in the roots and the above- 

 ground parts. The larger cells of the bundles, called 

 vessels, usually contain air. 



Finally a third modification induced by the change 

 from an aquatic to a terrestrial habitat, although some- 

 what later in its appearance than that of epidermis or 



