The separation of layers in the apical meristem of the root is usually very much more obvious than in that of the stem.
The branches of the stem arise by multiplication of the cells 01 the epidermis and cortex at a given spot, giving rise to a protuber ance, at the end of which an apical meristem is established.
If this division occurs by means of a localized secondary meristem connecting the cambial layers of adjacent bundles, an inlerfascicular is formed in addition to the fascicular cambium.
An ordinary cambium is scarcely ever found in the Monocotyledons, but in certain woody forms a secondary meristem is formed outside the primary bundles, and gives rise externally to a little secondary cortex, and internally to a secondary parenchyma in which are developed numerous zones of additional bundles, usually of concentric structure, with phloem surrounded by xylem.
In nearly all plants which produce secondary vascular tissues by means of a cambium there is another layer of secondary meristem arising externally to, but in quite the same fashion as, Ph II
The leaves always arise from the outer portion of the primary meristem of the plant, and the tissues of the leaf are continuous with those of the stem.
In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue, known as cambium; by the formation of a layer of cambium between the bundles (interfascicular cambium) a complete ring is formed, and a regular periodical increase in thickness results from it by the development of xylem on the inside and phloem on the outside.
Cernuum come nearest to this; in them the meristem forms a zone slightly below the summit, which may bear a number of green lobes.
Important points of difference exist, however, in the apical position of the meristem of the Ophioglossaceous prothalli, in the presence of a basal cell to the archegonium, and in the multiciliate spermatozoids.
It appears at first as a simple cellular papilla of meristem, upon which an indication of two lobes soon appears.