186 PRACTICAL LESSONS IN SCIENCE. 



contain protoplasm, but as they become older it disappears and 

 the tissue only contains air. 



These various tissues are grouped into three systems : the epi- 

 dermal, the fibro-vascular, and the fundamental. Neither of 

 these systems occur in the lowest plants. The epidermal system 

 is the most simple, and appears earlier in the life of the individ- 

 ual, and earlier in passing from lower to higher forms than the 

 other systems. The first indications of this system is in smaller, 

 darker colored surface cells in some of the lower fungi and their 

 allies, while in some of the mosses a distinct epidermis occurs, 

 becoming well developed in the ferns and flowering plants. The 

 epidermis proper consists of modified parenchyma, the cells 

 changing form, thickening and hardening their walls and finally 

 losing their protoplasm. 



From these epidermal cells hairs are produced, especially on 

 young roots, which vary somewhat in form and are active agents 

 m absorbing nutritive matters for the plant. And in the epider- 

 mis there are numerous stomata or breathing pores, which open 

 directly into intercellular spaces below. They are more numer- 

 ous on the under surface of leaves; on the lower surface of a 

 black walnut leaf there are nearly 300,000 stonmta in one square 

 inch of surface, with none on the upper side; while in the case of 

 a leaf of corn or maize, there are about 60,000 to the square 

 inch on the upper surface and about 100,000 on the lower sur- 

 face. In the case of the iris leaf and oat leaf there are more on 

 the upper surface. 



In most of the higher orders of plants the fibrous, sieve and 

 tracheary tissues, with some parenchyma, form bundles of com- 

 pact tissue running through the plant from roots to leaves. 

 They are called the fibro-vascular or vascular bundles. The 

 young radish is crisp and brittle the vascular bundles are not 

 noticeable; but later, as they are more fully developed, the rad- 

 ish becomes stringy and woody. They constitute the framework 

 of leaves and may be separated from the more delicate tissues by 

 maceration, and they are easily found as threads in the pith of 

 the cornstalk. The components of the bundles vary in their pro- 



