EPIDERMIS AND CORTEX. 119 



kind, the tissues of which are of course intimately united with 

 those of the main axis from which they are given off. 1 



PRIMARY STRUCTURE. 



359. In the stem, or ascending axis, the 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, except 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, Leyces- 

 teria formosa, and Ptelea trifoliata. 



In Palms 2 " 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 stony, brittle, shining layer." 



361. The primary cortex* consists essentially of parenchyma 

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

 such, for instance, as crystal 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. 



362. Immediately beneath the epidermis, and not easily dis- 

 tinguished from multiple epidermis, is a portion of the cortex 

 known as Hypoderma. 4 It is rarely sclerotic parenchyma, more 



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

 sequence of their development can be successfully followed out by the employ- 

 ment 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, Reports and Papers in Botany. The Palm-stem, 

 Henfrey's Translation, 1849, p. 14. 



3 Vesque (in Ann. des Sc. nat., ser. 6, tome ii., 1875, p. 82) gives a very full 

 treatment of the subject. 



4 The word Hvpoderma 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 Rindencollenchyms." It has since been ex- 

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



