i68 



PART II. — VEGETABLE HISTOLOGY. 



It differs considerably in different plants, and is not confined 

 to any particular portion of the plant, or to any tissue system, 

 but it is most common and most abundant in the parenchyma. 

 Two kinds are distinguished -the simple, in which the milk- 

 containing tubes consist of single, greatly elongated, and usually 

 branching cells, as in the Euphorbias and Ascle- 

 piads, Fig. 429, and the complex, in which they 

 consist of coalesced cells, forming an irregular 

 net-work of tubes, as in the Dandelion and other 

 Cichoraceae, Fig. 430. Closely allied, apparently, 

 to this latter variety of laticiferous tissue, are the 

 secretion cells of some plants, as those in the rhi- 

 zome of Sanguinaria. Here we find, in fact, every 

 gradation between isolated cells and chains of 

 cells that have coalesced into tubes, Figs. 431 

 and 432. 



Secretion Receptacles. These are not to be fig. 43 i. Fig. 43 2. 

 regarded as constituting a distinct tissue, since J 1G - 431— Row 



° of secretion cells 



for the most part they are but slightly modified from rhizome of 



J . Sanguinaria. 



parenchyma. It is convenient to classify them Magnified about 



75 diameters. 



according to their contents. . Fig. 432.— Por- 



. ,i hi 7 twn OI " m '" c tUDe 



Some contain crystals, and are called crystal from the same 



T ri 1 1 • " j plant, composed 



cells. In many of these the protoplasmic and of several conflu- 

 other contents have almost, if not completely, dis- M^g^fied^aboiit 

 appeared, and the cell is nearly or quite filled with 7S iameters - 

 the crystals. Though crystals are much the most abundant in 

 parenchyma cells, they are by no means confined to them. 



Other cells, on account of their resinous or balsamic con- 

 tents, are called resin cells ; others, containing mainly tannin, 

 are called tannin cells ; still others that are filled with volatile 

 oil are termed oil cells ; and those that contain an abundance of 

 mucilaginous or gummy matter are denominated mucilage cells. 



Primary Meristem. This term is applied to young and 

 forming cells, such as those found in the growing season between 

 the wood and the bark of Dicotyledons, at the growing apex of 

 the stem, root, etc. It is not to be regarded as a separate tissue, 

 but as made up of cells which are not yet differentiated or 

 developed into distinct tissues. It is always thin-walled, the cells 

 are relatively small, destitute of intercellular spaces, and in a 

 state of active division. 



