PROTECTIVE TISSUES. 



127 



only in the young embryo and in the tips 

 of growing plants, and all the rest is 

 secondary in its formation. 



It has been extensively taught that In 

 the apical region the primary meristem 

 gives rise to three layers of cells from 

 which the more complicated tissues 

 spring. The outer layer is called the 

 Dermatogen and gives rise in general to 

 the epidermal system of tissues. Be- 

 neath this lies the Flerome, which gives 

 rise to the fibro-vascular system, and 

 within is the Periblem from which the 

 ground tissues arise. This grouping is 

 adopted merely for the sake of conven- 

 ience, as it is not universal in plants 

 themselves. 



Much of the tissues that are built up 

 by the secondary meristem becomes per- 

 manent in form and makes the **Perma- 

 ncut tissue" of a former classification, 

 Drngs consist in general of permanent 

 tissues, but in the following meriste- 

 matic tissues may be found: Cambium 

 in Dulcamara stems, Radix Angelica and 

 Althaea and Secondary Meristem in 

 Jalap tubers. 'Apical cells may be found 

 in many herbs after treatment with di- 

 lute, ammonia or alcohol. Phellogen is 



to be found in oak bark, Inter fascicu- 

 lar Cambium in Riolnns stem, and Pri- 

 mary Cambium on the radicles and co- 

 tyledons of nearly all the seeds, such as 

 Capsicum, Croton, Ricinus, Sinapis, etc. 



CHAPTER VII. 



PROTECTIVE TISSUES 



The protective tissues of the plant con- 

 sist of the Epidermal and Mechanical 

 Systems, 



EPIDERMAL SYSTEM. The epi 

 dermal system is the outermost layer of 

 the plant and completely covers every 

 portion of it, protecting it from outside 

 mechanical injury and from too rapid 

 evaporation and loss of heat. 



In yonng plants the epidermal system 

 consists of the epidermis with its two 

 modifications, the hairs, or Trichomes, 

 and the Stomata. In many stems a year 

 or so old secondary changes take place 

 which lead to a new formation of tissues 

 which form the secondary epidermis. 

 The epidermis, or skin, is the superficial 

 la 3^er in roots, stems and leaves, and 

 consists of very variable cells. In the 



Fig. 27.— Belladonna leaves. Underside show- 

 ing epidermal cells, here slightly striped. sC, A, 

 and/i7, stomata. hair, and vessels respectively. 

 (Vogl). 



official cryptogams Cetrariu and Chon- 

 drus, the epidermis is not a true epider- 

 mis, but in Aspidium there is a well- 



marked superficial layer of epidermal 

 cells. 



The epidermal cells are generally flat- 

 tened and united to one another without 

 intercellular spaces. They are usually 

 elongated in the direction of the axis in 

 the monocotyledons and are irregular in 

 Dicotyledons. Often in seeds they are 

 thicker than . wide, but this is not the 

 rule. 



The outer wall of most epidermal cells 

 is thickened by a deposit of cutin which 

 renders it impervious to moisture and 

 also serves to protect it against insect 

 and fungus pests. This layer of cutin 

 varies greatly in thickness and bears a 

 direct ratio to the amount of transpira- 

 tion that serves the purpose of the indi- 

 vidual plant. In plants with leaves 

 more or less horizontal the side exposed 

 to the sun's rays is generally thicker. In 

 tropical countries where the heat is ex- 

 cessive and the loss of moisture would 

 be great the epidermis is much thicker 



than in temperate climates. 

 The modifications of the epidermis are: 

 In roots — Trichomes. 

 In stems— Trichomes, glands, stomata, 



water pores. 



Trichomes are modifications of the epi- 

 dermis and consist of outgrowths of a 

 single cell. They may be very simple, 

 hardly more than a slight projection, 

 or very complex, forming many-celled, 

 jointed and branched hairs. Their con- 

 tents may consist of protoplasm with a 

 cell nucleus, and sometimes crystals of 

 lime salts* 



