Ch. V, 10] SUMMARY OF PLANT TISSUES 265 



certain strands of procambium, which develop cell for cell 

 into the vascular bundles, while the tissue inclosed by them 

 forms the pith, that between them the medullary rays, 

 and that outside them the pericycle. If the procambium 

 cells all turn into xylem and phloem, we have a closed bundle 

 of the endogenous type, but if some of it turns into cam- 

 bium, we have open, continuously-growing bundles of the 

 exogenous type. In such cases the cambium usually 

 extends across the medullary rays and forms a continuous 

 cylinder. 



II. Functions performed in tissues requiring special posi- 

 tions but not forming morphological systems. 



7. Reproduction, which is asexual or sexual. If asexual, 

 the reproductive bodies are usually separable portions of thal- 

 lus or shoot. If sexual, the reproductive bodies, the mother 

 cells, spores, and sexual cells (later to be described), originate 

 in areas of primary meristem persistent on certain branches 

 (flowers). It requires a superficial position for fertilization 

 and dissemination, and hence the reproductive tissues are 

 developed in the cortex. 



8. Support of the foliage against strains of weight and 

 winds. It requires, upon well-known mechanical principles, 

 a tough, fibrous construction in a tube-form as near the 

 exterior as practicable ; but, to permit movement of air, water, 

 and food in and out through stems and roots, it is neces- 

 sarily discontinuous, and hence mostly disposed in strands. 

 The needs for this function vary greatly with the habits 

 of plants. Tissues, collenchyma, just beneath epidermis, 

 sclerenchyma in cortex or pericycle, bast fibers in phloem, 

 wood fibers in xylem, while special strengthening sclereids 

 occur in some exposed leaves. Because of the frequent 

 presence of fibers with the vascular bundles, especially in 

 herbaceous plants, these bundles are commonly designated 

 fibro-vascular, even if no fibers are present. Collectively 

 these tissues are sometimes described as forming a mechan- 

 ical system, though it has no morphological identity. 



