ao The Ottawa Naturalist. [April 



may frequently observe quite broad intercellular spaces, often to 

 such an extent as deserving the term " lacunes," which are very 

 common to roots of plants that grow in moist situations. 



The innermost layer of the cortex is differentiated into an 

 endodermis {End ), the structure and function of which suggests 

 that of the exodermis, and forms a closed sheath around the cen- 

 tral cylinder of the root. 



Bordering directly on the endodermis, thus representing the 

 outermost tissue of the cylinder, is a layer, and mostly a single 

 one, of thinwalled cells, which is called the pericycle or pericam- 

 bium (P.). The cell walls are never suberized nor do they show 

 any foldings. It is a tissue of great importance, since the lateral 

 roots become developed from this, and usually also the root- 

 shoots. 



Inside the pericycle we find the leptome (L. ) with sieve-tubes 

 and companion-cells, and the hadrome {H.) with the vessels. 

 These two elements, the leptome and hadrome, are in the root 

 arranged in separate groups, side by side, alternating with each 

 other in contrast to the stem, where they are located in the same 

 radius, the leptome outside, the hadrome inside. The vessels are 

 of different width in proportion to their age, the narrowest being 

 the earliest developed. The sieve-tubes and their companion-cells 

 are, as already stated, located between the rays of the hadrome, 

 and their delicate structure makes them readily distinguished 

 from the thick-walled vessels and conjunctive tissue, the last of 

 which occupies the centre of the root; it is parenchymatic and cor- 

 responds well with the pith of the stems. 



This root represents the annual type, and no increase in 

 thickness takes place, thus the root remains unchanged until it 

 dies at ihe end of the season. But if we now examine perennial 

 roots, we notice that an increase in thickness generally takes place 

 which results in greatly modified structures of which the following 

 is the most frequent and may be considered the normal. 



The first sign of change in structure is to be observed in the 

 central cylinder where a cambial tissue becomes formed in the 

 shape of arches and on the inside of the leptome ; this cambial 

 tissue thus originates in the conjunctive tissue bordering on the 



