404 PRIMARY ARRANGEMENT OF TISSUES. 



prevail as in that of the periphery of the pith. The peculiarities which come under 

 consideration in the case of stems with secondary formation of wood, are to be 

 compared in Chap. XIV. 



In most cases, and especially in those cases forming the rule, in which the stem 

 bears well-developed foliage-leaves, the ou/er cortex of the stem is built up of two more 

 or less distinct parts ; one, the Hypoderma ^, bordering directly on the epidermis, and 

 consisting of thicker-walled, closely united elements, which are often collenchymatous 

 or sclerotic, and in the latter case are still to be described in the following chapter ; 

 the other, a thinner-walled, infernal mass of parenchyma, the cells of which as a rule 

 become successively wider towards the interior, and always leave between them inter- 

 stices or lacunK filled with air. Both parts show a different arrangement according 

 to the particular cases. For the Dicotyledonous stem, Schleiden's^ classification may 

 be adopted, and the following principal forms may be distinguished. 



(a) The hypodermal layer surrounds the whole stem as a distinct, closed, multi- 

 seriate (collenchymatous) layer, which is only interrupted where stomata are present 

 by small gaps leading to the latter : many Cactese, Melianthus major, Euphorbia 

 splendens, Syringa vulgaris, Begonia macularis, Ailantus glandulosa, Rosa, Aris- 

 tolochia Sipho, Piper rugosum, Cacalia ficoides, and Cotyledon coccinea. 



(3) The collenchymatous or sclerotic hypoderma forms longitudinal bundles, 

 which alternate with longitudinal bands of thin-walled parenchyma, destitute of 

 collenchyma, reaching to the epidermis. The former usually occur in more or less 

 projecting corners of the stem, while the parenchymatous bands lie between them, 

 as in many angular stems, Umbelliferse, Chenopodiaceae, Malvaceae, Solanaceae, 

 and Sambucus ; in other angular stems, e.g. Labiatae, the collenchymatous hypo- 

 derma is massively developed in the angles, between them it is at least weaker in 

 every respect. 



(c) The epidermis is bordered by a collenchymatous hypoderma, which towards 

 the inside gradually passes over into the thinner-walled, loose mass of parenchyma, 

 and is broken up into isolated masses by the thin-walled parenchyma which extends 

 to the epidermis at the points where stomata occur. So in the primary cortex of 

 most Dicotyledonous woody plants, e. g. Pyrus, ^sculus, Salix, Cupuliferae, Betula, 

 Acer, Hedera, Tilia, etc., with more or less sharp limitation, and a varying number 

 of strata in the hypodermal layer. 



As follows directly from what has been said, the limitation and bulk of the 

 firmer hypodermal layer is very various, even in the preceding typical cases. In 

 weak stems, as in those of many water-plants, it often entirely disappears, and is 

 only indicated by the smaller width and closer connection of the cells bordering 

 directly on the epidermis, as compared with those which lie deeper. 



The converse case is more rare, namely that in which thin-walled, loose paren- 

 chymatous layers border immediately on the epidermis, while an inner zone forms 

 a many-layered sheath — then always more or less sclerotic — which surrounds the 

 ring of bundles ; e.g. species of Papaver and Thalictrum. 



On the occurrence of stomata, and their relations to the structural phenomena in 



' Compare p. 225. The word Hypoderma was introduced by Kraiis; Cycadeenfiedcrn, 

 Pringsheim's Jahrb. IV. 



* GiTindziige, 3 Aufl. II. p. 152. 



