EPIDERMIS AND CORTEX. 119 



kind, the tissues of wliieli are of course intimately united witli 

 those of the main axis from which they are given off.i 



Primauy Stuuctuke. 



359. In tlie stem, or ascending axis, tlie distribution of tissue 

 elements is similar to that in the descending axis, — the root. 

 There is a more or less transient epidermis, a cortical substra- 

 tum, and a central cylinder of some kind. 



360. The epidermis of stems presents few peculiarities of 

 structure bej'ond those already described in Chapter II. In 

 most herbaceous plants it persists with little change, e>:cept in 

 the matter of trichomes, throughout the life of the plant ; but 

 in most ligneous plants it is replaced, often early, by other pro- 

 tective tissues. Persistent epidermis is found in many woody 

 and half- woody plants ; for instance, Russelia juncea, Lej-ces- 

 teria formosa, and Ptelea trifoliata. 



In Palms ^ " the epidermis exists in old age only in the cane- 

 like and calamoid stems ; in the rest it is more or less destroyed 

 by the action of the weather. In Calamus it consists of a simple 

 layer of minute cells elongated in the direction from without 

 inward, and forms a ston}', brittle, shining layer." 



3G1. The primary cortex' consists essentiail}' of parenchj-ma 

 in which isolated cells of a peculiar character maj- often be found, 

 such, for instance, as crj'stal cells, laticiferous cells, tannin cells, 

 and the like (see 292) ; and its intercellular spaces sometimes 

 serve as receptacles for the various exudations. The paren- 

 chyma cells generally contain more or less chlorophyll, and some 

 starch. 



3G2. Immediatelj' beneath the epidermis, and not easilj- dis- 

 tinguished from multiple epidermis, is a portion of the cortex 

 known as Hypoderma.* It is rarely sclerotic parenchyma, more 



' In the plumule and other buds all these parts exist potentially ; and the 

 sequence of their development can be successfully followed out by the enii>loy- 

 mcnt of seeds in different stages of germination, or buds collected on succes- 

 sive days in spring and preserved at once in alcohol. In all cases care must 

 be taken to have the date of collection of each specimen recorded in such a 

 manner that no confusion can afterwards arise. 



2 Mohl : Ray Society, Eeports and Papers iu Botany. The Palm-stem, 

 Heufrey's Translation, 18-19, p. 14. 



■^ Vesque (iu Ann. des Sc. nat., sdr. C, tome ii., 1875, p. 82) gives a very full 

 treatment of the subject. 



* The word Hypoderma was introduced by Kraus (Pringsheim's Jahrb., 

 1865-66, p. 321), to designate the layer of colorless cells under the epidermis 

 of leaves, "das Analogon des Uindencollenchyms." It has since been ex- 

 tended to apply to the external cortex just under the epidermis of stems. 



